But if you want to replace one wife with another and you (4:20)

﴿وَإِنْ أَرَدتُّمُ اسْتِبْدَالَ زَوْجٍ مَّكَانَ زَوْجٍ وَآتَيْتُمْ إِحْدَاهُنَّ قِنطَارًا فَلَا تَأْخُذُوا مِنْهُ شَيْئًا ۚ أَتَأْخُذُونَهُ بُهْتَانًا وَإِثْمًا مُّبِينًا﴾
[ النساء: 20]

4:20 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?

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.

But if you intend to replace a wife by another and you have given one of them a Cantar (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?

phonetic Transliteration

Wain aradtumu istibdala zawjin makana zawjin waataytum ihdahunna qintaran fala takhuthoo minhu shayan atakhuthoonahu buhtanan waithman mubeenan

Abdullah Yusuf Ali - Translation

But if ye decide to take one wife in place of another, even if ye had given the latter a whole treasure for dower, Take not the least bit of it back: Would ye take it by slander and manifest wrong?

Safi-ur-Rahman al-Mubarakpuri

But if you intend to replace a wife by another and you have given one of them a Qintar, take not the least bit of it back; would you take it wrongfully without a right and (with) a manifest sin

Page 81 English transliteration