The agreement is valid. A private written agreement is a reconciliation after a dispute occurs. Your private agreement has been signed by both parties;
 It is the free and true declaration of intention of both parties, which has legal effect. If the other party reneges, it can file a lawsuit to the court according to this written agreement.
 [Legal Basis]
 In Article 490 of the Civil Code, if the parties conclude a contract in the form of a contract, the contract is formed when all the parties sign, seal or fingerprint.
 The contract is formed when one party has performed its main obligations and the other party has accepted them before signing, sealing or fingerprinting.
 A contract shall be concluded in writing as required by laws and administrative regulations or as agreed by the parties. A contract is formed when the parties fail to adopt a written form but one party has performed its main obligations and the other party has accepted it.