Owe money

Assalaam U Alaikum Mufti Elias

if i loan money to someone, and make specific agreement with him, if he returns the money and adds to it on his own accord is this viewed as interest ? or if he buys me a gift or an item as a token of gratitude will this be viewed as interest ?

no specific agreement in place, it is based on his own free will.

Please advise.

Jazakallah