What is the ruling on deceiving someone who delays in repaying a debt by agreeing with him to do work for a payment equal to the amount owed, and then not paying for the work on the pretext that the amount is a previous debt?

There is no blame upon you for demanding the same amount from the debtor, even if you are forced to lie to obtain your right. Ibn al-Qayyim held the view that lying is permissible if one can attain their right through it, and it does not involve harming others. Lying is permitted in cases such as reconciling people, during war, or for the wife. You are also permitted to take your right from the debtor's money without his permission, which is known as the issue of al-Zafar (taking what is rightfully yours).