It should be understood that it is permissible for a borrower to voluntarily or out of good will repay more than the amount originally borrowed, provided that no condition, agreement or expectation was made at the time of the loan requiring any amount in excess of the principal.
In fact, repaying a debt in a generous manner is not only permissible but is also an act of noble character and a Sunnah practice.
Therefore, in the scenario described, since there was no prior condition or agreement to pay anything beyond the principal, it is permissible for the borrower to voluntarily give an additional amount out of goodwill. Such an additional payment will not be regarded as ribā (interest), as it is purely voluntary and not a stipulated benefit of the loan.
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دلائل:
مرقاة المفاتيح: (1954/5، ط: دار الفکر)
وَفِيهِ دَلِيلٌ عَلَى أَنَّ مَنِ اسْتَقْرَضَ شَيْئًا فَرَدَّ أَحْسَنَ أَوْ أَكْثَرَ مِنْهُ مِنْ غَيْرِ شَرْطِهِ كَانَ مُحْسِنًا وَيَحِلُّ ذَلِكَ لِلْمُقْرِضِ
صحیح مسلم: (رقم الحدیث: 1601، ط: دار احیاء التراث العربی)
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارِ بْنِ عُثْمَانَ الْعَبْدِيُّ. حدثنا محمد بن جعفر. حدثنا شعبة عن سَلَمَةَ بْنِ كُهَيْلٍ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ. قَالَ:
كَانَ لِرَجُلٍ عَلَى رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ حَقٌّ. فَأَغْلَظَ لَهُ. فَهَمَّ بِهِ أَصْحَابُ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ. فَقَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ (إِنَّ لِصَاحِبِ الْحَقِّ مَقَالًا). فَقَالَ لَهُمْ (اشْتَرُوا لَهُ سِنًّا فَأَعْطُوهُ إِيَّاهُ). فَقَالُوا: إِنَّا لَا نَجِدُ إِلَّا سِنًّا هُوَ خَيْرٌ مِنْ سِنِّهِ. قَالَ (فَاشْتَرُوهُ فَأَعْطُوهُ إِيَّاهُ. فَإِنَّ مِنْ خَيْرِكُمْ - أَوْ خَيْرَكُمْ - أَحْسَنُكُمْ قَضَاءً).
Allah Almighty knows best,
Darul ifta Al-Ikhlas, Karachi.