Allah said:

﴿وَلاَ يَحِلُّ لَكُمْ أَن تَأْخُذُواْ مِمَّآ ءَاتَيْتُمُوهُنَّ شَيْئًا﴾

(And it is not lawful for you (men) to take back (from your wives) any of (the dowry) what you gave them,) meaning, you are not allowed to bother or pressure your wives to end this situation by giving you back the Mahr and any gifts that you have given them (in return for divorce). Similarly, Allah said:

﴿وَلاَ تَعْضُلُوهُنَّ لِتَذْهَبُواْ بِبَعْضِ مَآ ءَاتَيْتُمُوهُنَّ إِلاَّ أَن يَأْتِينَ بِفَاحِشَةٍ مُّبَيِّنَةٍ﴾

(...and you should not treat them with harshness, that you may take away part of what you have given them, unless they commit open illegal sexual intercourse.) (4:19)

However, if the wife willingly gives back anything with a good heart, then Allah said regarding this situation:

﴿فَإِن طِبْنَ لَكُمْ عَن شَىْءٍ مِّنْهُ نَفْساً فَكُلُوهُ هَنِيئاً مَّرِيئاً﴾

(...but if they, of their own good pleasure, remit any part of it to you, take it, and enjoy it without fear of any harm.) (4:4)