What should I do if the other party refuses to give money or even write an iou after the project without a contract is over?

According to the contract law, even if no contract is signed, if one party pays, the other party will accept it by default. It can also be regarded as the signing of the contract. You have kept enough evidence to prove that you have paid the labor, but the other party has not paid the remuneration. If the other party says that the remuneration has been paid, it is legally necessary to provide proof of payment. If he does not provide proof of payment, no salary will be paid.