Legal analysis: Modification of the contract requires agreement between both parties, otherwise it is illegal and the modification is invalid. Generally, it is a civil liability. If the developer attempts to misappropriate other people's legitimate property by tampering with the contract, it may be suspected of fraud and need to bear corresponding legal liability. You can consult a lawyer and file a lawsuit in court to ask the developer to withdraw the contract that was modified privately, or to ask the developer to provide a certain amount of compensation.

Legal basis: "The People's Republic of China and the Civil Code"

Article 543 The parties may modify the contract if they reach consensus through consultation.

Article 119 A contract established in accordance with the law shall be legally binding on the parties.