Is saying "Jazakallahu Khairan" to someone from whom I took money considered forbidden?

The Prophet, peace and blessings be upon him, commanded that whoever does a favor should be rewarded. If we do not find anything to reward them with, then we should supplicate for them.

The best way to reward a favor is by choosing something that brings joy to the person who did the favor.

One of the best forms of supplication is "May Allah reward you with good (Jazakallahu Khairan)," as it conveys extensive praise and a prayer for abundant good. This has been reported in the Sunnah and on the tongues of the Companions, because if Allah rewards him with good, it will bring him happiness in this world and the Hereafter.

The reward should be according to the situation. For some people, the reward is a gift, and for others, it is by supplicating for them, such as saying, "May Allah reward you with good (Jazakallahu Khairan)."