If I deceive him who deceived me, am I sinful?

The general principle regarding deceit is the permissibility of retaliation in kind, based on the Almighty's saying: "So whoever has assaulted you, then assault him in like manner as he assaulted you" (Al-Baqarah: 194).

Thus, whoever permits your blood, his blood is permissible by the ruler's judgment. And whoever takes your wealth, it is permissible to take his wealth if it is of the same type as yours and you are able to do so. If it is not of the same type as yours, the more correct view is that you should ascertain its value and take an equivalent amount.

If he takes your honor, it is permissible for you to take his honor in kind without exceeding bounds. However, you are not permitted to lie to him, even if he lied to you, because a sin is not countered with a sin. If he says to you: "O disbeliever," it is permissible for you to say to him: "You are the disbeliever." If he delays paying you while he is wealthy and without excuse, then say: "O oppressor, O one who devours people's wealth."

As for someone who denies a deposit, it is permissible to deny him in kind. If he deposited one hundred with you, and you deposited fifty with him, and he denied the fifty, it is permissible for you to deny him fifty like it.