In the absence of performance or payment, a subsidiary letter can only become legally binding if it is executed as an instrument, which implies, inter alia, that it must indicate that the subsidiary letter is an act and that the signatures of the parties must be signed. For a secondary letter to become mandatory, it must meet the same criteria as all contracts: (i) the offer; (ii) adoption; (iii) recital (iv) certainty; and (v) the intention to create legal relationships. First of all, it is forbidden to use side letters to change the price of a real estate transaction. Since part of the overall price would be included in the subsidiary letter, it would make it possible to indicate a lower price in the primary contract, which would reduce the taxable base. In that situation, the subsidiary letter would not be valid and, in addition, would expose the parties to the subsidiary letter and the professional author to sanctions. Third, by changing the terms of the main contract, the secondary letter may change the characterization of the entire agreement. For example, in France, some management leasing contracts are concluded by the parties to obtain the effective qualification of a contract, that is to say. The purchase of the company, in the secondary letter. . . .