The Quran

وَإِن فاتَكُم شَيءٌ مِن أَزواجِكُم إِلَى الكُفّارِ فَعاقَبتُم فَآتُوا الَّذينَ ذَهَبَت أَزواجُهُم مِثلَ ما أَنفَقوا ۚ وَاتَّقُوا اللَّهَ الَّذي أَنتُم بِهِ مُؤمِنونَ

Ali Unal

If anything of the bridal-dues of your (former) disbelieving wives (who remain among the unbelievers u or have joined them) has passed to the unbelievers, and afterwards you have your turn (of victory) over them, then pay to those whose wives have gone away the equivalent of what they expended (as bridal-due).¹ And keep from disobedience to God in reverence for Him and piety, in Whom you are believers.

If the unbelievers did not return what the Muslim husbands had spent on their (former disbelieving) wives as dowry, the Muslims were to compensate this out of what the (former disbelieving) husbands had spent on their wives who had converted to Islam. Or, if the Muslims were to gain war-spoils from the unbelievers, the Muslim husbands were to be compensated out of this.