4:20
وَإِنْ أَرَدتُّمُ ٱسْتِبْدَالَ زَوْجٍ مَّكَانَ زَوْجٍ وَءَاتَيْتُمْ إِحْدَىٰهُنَّ قِنطَارًا فَلَا تَأْخُذُوا۟ مِنْهُ شَيْـًٔا ۚ أَتَأْخُذُونَهُۥ بُهْتَـٰنًا وَإِثْمًا مُّبِينًا
wa-in aradttumu is'tib'dāla zawjin makāna zawjin waātaytum iḥ'dāhunna qinṭāran falā takhudhū min'hu shayan atakhudhūnahu buh'tānan wa-ith'man mubīnan
If you wish to replace one wife with another, do not take any of her bride-gift back, even if you have given her a great amount of gold.
But if you intend to replace a wife by another and you have given one of them a Qintâr (of gold i.e. a great amount as Mahr) take not the least bit of it back; would you take it wrongfully without a right and (with) a manifest sin?