What is the ruling on lying to one's brother and agreeing with a worker to tell him a repair price lower than the real price, in order to avoid problems with him, knowing that the money paid is the worker's right?

If avoiding evil from the brother can only be achieved by informing him of something contrary to reality, then we hope that there is no sin upon you. The general principle is that lying is not permissible unless the praiseworthy objective cannot be achieved except through it, along with the necessity of seeking forgiveness and repentance from any sin.