The Quran

يا أَيُّهَا الَّذينَ آمَنوا إِذا جاءَكُمُ المُؤمِناتُ مُهاجِراتٍ فَامتَحِنوهُنَّ ۖ اللَّهُ أَعلَمُ بِإيمانِهِنَّ ۖ فَإِن عَلِمتُموهُنَّ مُؤمِناتٍ فَلا تَرجِعوهُنَّ إِلَى الكُفّارِ ۖ لا هُنَّ حِلٌّ لَهُم وَلا هُم يَحِلّونَ لَهُنَّ ۖ وَآتوهُم ما أَنفَقوا ۚ وَلا جُناحَ عَلَيكُم أَن تَنكِحوهُنَّ إِذا آتَيتُموهُنَّ أُجورَهُنَّ ۚ وَلا تُمسِكوا بِعِصَمِ الكَوافِرِ وَاسأَلوا ما أَنفَقتُم وَليَسأَلوا ما أَنفَقوا ۚ ذٰلِكُم حُكمُ اللَّهِ ۖ يَحكُمُ بَينَكُم ۚ وَاللَّهُ عَليمٌ حَكيمٌ

(Saheeh International)

O you who have believed, when the believing women come to you as emigrants, examine [i.e., test] them. Allāh is most knowing as to their faith. And if you know them to be believers, then do not return them to the disbelievers; they are not lawful [wives] for them, nor are they lawful [husbands] for them. But give them [i.e., the disbelievers] what they have spent.¹ And there is no blame upon you if you marry them when you have given them their due compensation [i.e., mahr]. And hold not to marriage bonds with disbelieving women, but ask for what you have spent and let them [i.e., the disbelievers] ask for what they have spent.² That is the judgement of Allāh; He judges between you. And Allāh is Knowing and Wise.

For marriage, i.e., compensate their loss.
When a disbelieving wife chose to join the disbelievers, a Muslim husband could demand in return the equivalent of her mahr. Likewise, the disbelievers had a similar right when a believing woman joined the Muslims. This and the following verses were revealed subsequent to the Treaty of al-Ḥudaybiyyah.