If someone extends a free loan "Qardh Al-Hasanah" as a gesture of help without any conditions and the borrower, while using it in business, realizes a handsome profit, will be right if he returns the loan with a part of the realized profit, in reciprocation of his sincere help.

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.

There is no harm in this unless stipulated in the agreement. In fact, a good payment means giving better than what was taken, and it is lawful as long as it is not stipulated in the contract of the loan.

Muslim reported that the Prophet borrowed a small camel from a man then when the Zakaah camels came to him, he ordered Raafi’ to pay the man back his camel. Raafi’ found only big camels then he told the Prophet . The Prophet said to him: “Give him (this man) a big camel. Indeed, the best among all people are those who pay back in the best way.”

Allah knows best.