Valid. If the parties conclude a contract in the form of a contract, the contract is established when both parties sign or seal it.

Article 490 of the Civil Code: Where the parties conclude a contract in the form of a contract, the contract is established when both parties sign, seal or fingerprint the contract. Before signing, sealing or fingerprinting, one party has performed its main obligations and the contract is established when the other party accepts it.

Laws and administrative regulations or the parties agree that a contract should be concluded in writing. If the parties do not use writing but one party has performed its main obligations and the other party accepts it, the contract is established.