Crypto-assets are not currencies, at most they are means of payment. As a means of payment that is not legal tender, forced payment in crypto-assets is impossible, unless the contract of sale is requalified as a contract of exchange (I.). However, a contract of sale may be terminated by the transmission of crypto-assets, provided that this method of payment is agreed to by the creditor of the monetary obligation (II.).
I. MANDATORY PAYMENT IN CRYPTO-ASSETS
It is not possible to denominate a monetary obligation in crypto-asset units, since a sum-of-money obligation is denominated in euro, or conditionally, in another recognised currency. This is not the case with crypto-assets, which can nevertheless influence the amount of the obligation (A.). Since an obligatory payment is not permitted from the angle of the monetary obligation, the transmission of crypto-asset units as the principal obligation can only be analysed from the angle of the exchange (B.).
A. THE DIFFICULT LIBEL OF A MONETARY OBLIGATION IN CRYPTO-ASSETS
The obligations are varied, but follow a common regime regarding their extinction by payment. However, monetary obligations are subject to special rules, reserved for legal tender, so that payment of a monetary obligation is made in euro, imposing its use as the unit of account for any payment of a monetary obligation.
The principle of monetary nominalism - Monetary nominalism is defined as "the principle according to which a monetary unit, as long as it has the same name, retains the same (discharging) value, even if over time its real value (purchasing power) has changed, and by application of which the debtor of a certain quantity of monetary units always owes the same numerical sum, without revaluation". In other words, the monetary unit is invariable over time, and a claim cannot, in principle, be revalued. This principle is repeated in Article 1343, paragraph 1 of the Civil Code, which provides that "the debtor of an obligation to pay a sum of money is discharged by the payment of its nominal amount". Prior to the 2016 reform, Article 1895 of the Civil Code provided that: "The obligation resulting from a cash loan is always only the sum stated in the contract". The provision originally concerned only loans of money, but its extension to all contracts by case law meant that it had to be transposed to the general regime of obligations. However, this principle has no constitutional value, nor even the character of a measure of public policy, so that it allows contrary stipulations.
Value debt - Since monetary nominalism is an exceptional principle, its counterpart is value debt. The debt of value finds its origin in Article 1343, paragraph 2 of the Civil Code, which states that "the debtor of a debt of value is discharged by the payment of the sum of money resulting from its liquidation". As an exception to the principle of monetary nominalism, the debt of value is, like the nominal obligation, a debt which is discharged by the payment of a sum of money, but the amount of which is fixed not in advance, "but according to a real value, assessed at maturity". However, most of the applications of the debt of value concern legal hypotheses such as tort liability (the evaluation of the damage must take place on the day of the judgment, and not on the day when it occurred), matrimonial property regimes (for the calculation of the rewards due to the community) or joint and several liability. Thus, in these rare, but nonetheless probable, cases, the infringement of monetary nominalism could take into account changes in the values of crypto-assets and weight changes for which the payment of the nominal would be irrelevant, without being able to be used in the contractual sphere.
Indexation of the value of the monetary bond - If it is not possible to denominate a bond in crypto-asset units, the value of the latter seems to allow it to be used as an index of a monetary bond, under strict conditions. Thus, if a bond cannot be denominated in crypto-asset units, its "amount may vary through indexation". The debt is said to be indexed. Claims are indexed when the money bond is linked to an objective reference, the index, which tends to vary its amount. It differs from price revision, which is a new agreement, so that the legal mechanism is based on novation, whereas indexation does not generate any novation, because even when the object changes, the contract remains the same as it was originally. It may be obtained ipso jure, judicially (mainly for debts of maintenance and compensatory allowance paid in the form of a life annuity) or by agreement. It is in the latter type of indexation that the use of crypto-assets as an index seems to be expressed.
Indexation clause or foreign currency? - It has already been said, but Article 1343-3 of the Civil Code requires the unit of payment on French soil to be the euro. Nevertheless, like any principle, "payment may be made in another currency if the obligation so expressed arises from an international contract or a foreign judgment". Without being a real currency, could crypto-assets constitute a currency? The question refers to the previous developments concerning the monetary nature of these units. If this were the case, merchants who would decide to denominate their prices in such units would then stipulate a foreign currency clause. Indeed, the parties may fix the wording of the monetary obligation of a domestic contract in a foreign currency as long as the payment is made in its equivalent value in euro. The stipulation of an obligation in a foreign currency is lawful if that currency is provided for as a payment instrument but not as a unit of account. Such a provision is interpreted as an indexation clause subject to Article L112-2 of the Monetary and Financial Code. This stipulation is valid if the chosen benchmark is directly related to the purpose of the agreement or to the activity of one of the parties. There must necessarily be a de facto relationship between the index and the subject matter of the agreement, otherwise it is qualified as disguised indexation in internal contracts. Failing this, indexation is unlawful and renders the clause, or even the whole agreement, absolutely null and void if the clause is essential to its existence. In order to be legally viable, this hypothesis presupposes that crypto-assets are recognised in France as foreign currencies and, if this could have been argued by the CJEU, the French position would have been different. However, the absence of recognition as a currency does not totally prevent crypto-assets from being the subject of such clauses, as evidenced by the admission of the use of the ecu, which is not recognised as a currency but whose use was confirmed by a simple law authorising its use and which confirmed the validity of an index clause using it as an index.
Possible sectoral tolerance - Despite these strict conditions, case law allows price indexation by reference to a foreign currency in credit activities. It is therefore lawful for a credit institution to index its loans to a foreign currency, even for a purely domestic transaction. It is conceivable that a debt may be denominated in crypto-asset as long as the purpose of the contract or the activity of one party justifies it and payment is made in euros. Thus, if one of the parties uses crypto-assets, or is a company in this sector, it would seem that such indexation could be lawful. Moreover, some argue that this exclusively French regulation would be incompatible with European law, which has normally been issued by the EU institutions since the changeover to the euro, without imposing such restrictions on the indexation clauses.
B. THE EXCHANGE CONTRACT AS A MANDATORY METHOD OF PAYMENT
Since a monetary obligation can only be denominated in a currency that is legal tender in a State, and since payment can only be made in euros, it is impossible to make it compulsory for the seller of a good, in France, to make a payment in crypto-asset. If he wishes to obtain only crypto-assets in the exchange of his good, for lack of monetary obligation, the contract will necessarily be an exchange. The obligation of transmission of ownership of the units will not be analyzed as a monetary obligation, but a simple obligation of transfer of ownership. Consequently, the fact of imposing crypto-assets as a means of payment will not be analysed as a contract of sale, but as a contract of exchange.
The exchange contract - Legally enshrined in Title VII of the Civil Code, the exchange contract is the "contract by which the parties respectively give one thing for another. » . The exchange from euro to crypto-asset, if it is exempt from VAT is not an exchange, since one of the parties must provide a monetary obligation. Similarly, the transfer of a crypto-asset will be a sale. The contract will only really be an exchange when one person offers to transfer ownership of a good to another person in return for x units of a particular crypto-asset, without agreeing to transfer ownership by paying the value of the corresponding euro units. The hypothesis of an exchange of crypto-assets, i.e. the contract whereby one person would transmit x units of a crypto-agent in exchange for x units of another crypto-agent, still needs to be addressed. While it is tempting to think that, like the exchange of goods for crypto-assets, the legal form chosen is exchange, since the structure of this market is similar to a stock market, this solution does not seem to be feasible in all cases. Indeed, even simultaneous operations of purchase and sale of securities on the stock exchange cannot be considered as an exchange of securities. Thus, it would be advisable to limit the exchange contract system to one-off transactions without an intermediary, or to check each transaction to determine the will of the parties.
Regime of the exchange contract - As with almost any transfer of ownership, "the goods being exchanged must necessarily be the property of the contracting parties". Failing this, the party having received the thing in dispute is bound to return it. A guarantee of eviction exists with regard to the evicted co-perpetrator: he has a choice between an award of damages and the repetition of the thing. A contract that is fairly close to a sale in its operation, the exchange "takes place by consent alone", and "the rules prescribed for the contract of sale are applicable". However, and unlike the contract of sale, "rescission on account of injury does not take place in the contract of exchange. » .
Thus, despite the detour through the exchange contract, the obligation to pay something in crypto-asset seems legally viable, although more difficult than an optional payment.
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French legal analysis of payment in crypto-currencies – An usefull alternative method of payment 1/2
 G. Cornu et Association H. Capitant, « Vocabulaire juridique », éd. PUF, 19ème éd.
 N. Cayrol, « Synthèse 680 : Monnaie de paiement », JurisClasseur Civil Code (Maj, 11 déc. 2017), par. 37.
 Cass. req., 25 nov. 1929 : DH 1929, p. 161 ; Gaz. Pal. 1929, 1, p. 802 ; Cass. req., 25 oct. 1932 : DH 1932, p. 556 ; Gaz. Pal. 1933, 1, p. 36.
 N. Cayrol, « Synthèse 680 : Monnaie de paiement », JurisClasseur Civil Code (Maj, 11 déc. 2017), par. 38.
 Cons. const., 16 janv. 1982, n° 81-132 DC, JORF, 17 janv. 1982, p. 299.
 Cass. 1re civ., 27 juin 1957 : JurisClasseur Périodique Général, 1957, II, 10093 bis, concl. A. Besson ; Revue Trimestrielle de Droit civ. 1957, p. 553, obs. J. Carbonnier.
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 Responsabilité délictuelle : Cass. req., 24 mars 1942 : Gaz. Pal. 1942, 1, p. 224 ; responsabilité contractuelle : Cass. soc., 18 juin 1953 : Gaz. Pal. 1953, 2, p. 240, note Mazeaud.
 Art. 815-13, C. civ. providing that the compensation which may be due to an undivided co-owner who has improved an undivided property at his own expense must be fixed in equity "having regard to the increase in the value of the property".
 The assessment of the damage resulting from a fault in the management of crypto-assets should then be assessed on the day of judgment.
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 Case law does not see indexation in clauses which provide for the revision of the expert's quoted price (Com., 16 March 1966: JCP G 1966, IV, 66; Com., 20 Jan. 1970: Bull. civ. IV, No 27), or by negotiation between the parties (Soc., 30 Apr. 1985: Bull. civ. V, No 267; JCP G 1985, IV, 244).
 Notably for pensions constituted with insurance companies (see L. No. 49-1098, 2 August 1949, revising certain life annuities constituted by insurance companies, by the National Retirement Pension Fund for Old Age or by private individuals through the alienation of capital in cash, JORF, 6 August 1949, and L. No. 51-695, 24 May 1951, increasing certain life annuities and pensions, JORF, 5 June 1951) or awarded as compensation for damage caused by a road accident (see L. No. 74-1118, 27 Dec. 1974, relating to the revaluation of certain annuities awarded as compensation for damage caused by a land motor vehicle, JORF, 28 Dec. 1974).
 Art. 208, al. 2, C. civ.
 Art. 276-1, al. 1er, C. civ.
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 Voir infra. par. 38-44.
 Civ. 1ère, 25 mars 1981, n° 79-16.847, Bull. civ. I, n° 104 :"Whereas the stipulation of an obligation in foreign currency is lawful as soon as this currency is provided for, not as an instrument of payment, but as a unit of account".
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 Civ. 3ème, 13 février 1969, Bull. n° 134 : JCP 1969, II 15942., (2e esp.).
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 Civ. 1ere, 13 avr. 1999 : Bull. civ. I n°135 ; Recueil Dalloz, 2000. Somm. 365 obs. R. Libchaber.
 Com., 22 mai 2001, n° 98-14.406, Bull. civ. I, n° 98, p.91 ; Civ. 1ère, 29 mars 2017, n° 16-13.050, Bull. : Recueil Dalloz 2017, 1893, note C. Kleiner.
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 N. Cayrol, « Synthèse 680 : Monnaie de paiement », JurisClasseur Civil Code (Maj, 11 déc. 2017), par. 47.
 M. Rousille, « Le bitcoin : objet juridique non identifié », éd. Banque & Droit nº 159 janv./févr. 2015, p. 30.
 Art. 1702, C. civ.
 CJUE 22 oct. 2015, aff. C-264/14, « Skatteverket c/ Hedqvist, Skatteverket c/ David Hedqvist ».
 Com., 3 mai 1973, n°71-10.587, Bull. civ IV, n°158, p. 137.
 Art. 1704, C. civ.
 Art. 1705, C. civ.
 Art. 1703, C. civ.
 Art. 1707, C. civ.
 Art. 1706, C. civ.