Answer
(Fatwa: 156/156/M=1432)
It is lawful to pay more than the borrowed money provided there is no condition of extra amount and the creditor does not harbour a desire it to receive extra amount, rather it is good, and proved by hadith.
“إن خياركم أحسنكم قضاء” و فى رواية عن جابر قال : كان لى على النبى صلى الله عليه وسلم دين ، فقضالي و زادنى (مشكوة)
However, at the time of paying the debt one should say like this: this is your capital demand and this extra amount is from my side.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband