What should I do about the engineer who did not complete the apartment's finishing, leaving it with irreparable defects, and now does not want any payment? Should I give him the full agreed-upon amount, a portion of it, or nothing at all? If a portion, how much should I give him, and what should I do if he refuses?

The questioner has the right to deduct from the agreed-upon amount with the engineer what is equivalent to the deficiency in his work that he did not complete, and the value of defects in workmanship resulting from his negligence. The estimation of this is referred to experts, and if they do not reach an agreement, then disputed matters are resolved by resorting to the judiciary or arbitration councils.