Assalaamu alaykum. I lent some money to someone many years ago, but that person has not returned my money, but now I have come across some money that belongs to that person (which is less than 1/5 of the money I lent him). Can I keep that money as a return of the money which I lent him without telling him? I know that this person will never give my money back; even though he is wealthy, he keeps giving excuses. Would I be sinning if I kept this money without telling him? May Allah reward you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

What you are asking about is the issue of Thafar (getting one’s right back secretly): whether or not it is permissible for you to take money to the extent of your right only, without any transgression, from the person who wronged you. The scholars discussed this issue, and the preponderant opinion is that it is permissible for a wronged person to take his right without any excess.

Imaam Al-Qurtubi said in his interpretation of the saying of Allah (which means): {Indeed, Allah commands you to render trusts to whom they are due...} [Quran 4:58], "The correct opinion is that it is permissible for him to do so in any manner in which it is possible for him to get back his right as long as he is not considered a thief; this is the view of Ash-Shaafi‘i, and Ad-Daawoodi reported this from Maalik. Ibn Al-Munthir also stated it, and it is the view chosen by Ibn Al-‘Arabi, and that this is not deceit; rather, it is getting one’s right back..."

Ash-Shaafi'i said in Al-Umm, "The Sunnah and the consensus of many scholars indicate that a man is entitled to take back his right in secret from the one who wronged him; it was proven that this is not deceit. Deceit is taking what is not lawful to take. If a person deceives me by taking one dirham from me, and I say: 'He considered deceiving me as lawful,' this is not a reason for me to take 10 dirhams from him as a requital for deceiving me; I am only entitled to take one dirham, and by doing so, I am not deceiving or doing an act of injustice."

Based on this, if that man is procrastinating and preventing to give you back your right and you cannot get your right back from him, and then you obtain some of your money from his money (wealth) and you took it, then we hope that there is no harm on you for doing so. But you should inform him about what happened so that he would know that he is acquitted from the amount of money which you took from him.

Allah knows best.