Allah said:

﴿ظˆظژظ„ط§ظژ ظٹظژط­ظگظ„ظ‘ظڈ ظ„ظژظƒظڈظ…ظ’ ط£ظژظ† طھظژط£ظ’ط®ظڈط°ظڈظˆط§ظ’ ظ…ظگظ…ظ‘ظژط¢ ط،ظژط§طھظژظٹظ’طھظڈظ…ظڈظˆظ‡ظڈظ†ظ‘ظژ ط´ظژظٹظ’ط¦ظ‹ط§﴾

(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:

﴿ظˆظژظ„ط§ظژ طھظژط¹ظ’ط¶ظڈظ„ظڈظˆظ‡ظڈظ†ظ‘ظژ ظ„ظگطھظژط°ظ’ظ‡ظژط¨ظڈظˆط§ظ’ ط¨ظگط¨ظژط¹ظ’ط¶ظگ ظ…ظژط¢ ط،ظژط§طھظژظٹظ’طھظڈظ…ظڈظˆظ‡ظڈظ†ظ‘ظژ ط¥ظگظ„ط§ظ‘ظژ ط£ظژظ† ظٹظژط£ظ’طھظگظٹظ†ظژ ط¨ظگظپظژط§ط­ظگط´ظژط©ظچ ظ…ظ‘ظڈط¨ظژظٹظ‘ظگظ†ظژط©ظچ﴾

(...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:

﴿ظپظژط¥ظگظ† ط·ظگط¨ظ’ظ†ظژ ظ„ظژظƒظڈظ…ظ’ ط¹ظژظ† ط´ظژظ‰ظ’ط،ظچ ظ…ظ‘ظگظ†ظ’ظ‡ظڈ ظ†ظژظپظ’ط³ط§ظ‹ ظپظژظƒظڈظ„ظڈظˆظ‡ظڈ ظ‡ظژظ†ظگظٹط¦ط§ظ‹ ظ…ظ‘ظژط±ظگظٹط¦ط§ظ‹﴾

(...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)