Surah An-Nisa — Ayah 20 (4:20)
وَإِنۡ أَرَدتُّمُ ٱسۡتِبۡدَالَ زَوۡجٖ مَّكَانَ زَوۡجٖ وَءَاتَيۡتُمۡ إِحۡدَىٰهُنَّ قِنطَارٗا فَلَا تَأۡخُذُواْ مِنۡهُ شَيۡـًٔاۚ أَتَأۡخُذُونَهُۥ بُهۡتَٰنٗا وَإِثۡمٗا مُّبِينٗا ٢٠
English translation (Saheeh International)
But if you want to replace one wife with another and you have given one of them a great amount [in gifts], do not take [back] from it anything. Would you take it in injustice and manifest sin?
Brief commentary
If you want to divorce a woman and replace her with another, then you are allowed to do so; but if you gave the one you intend to divorce a great deal of wealth as a dowry, then you are not allowed to take anything back from it. Taking back what you gave to them would be an injustice and a clear sin.