Al-Mumtahana 60:10

O you who believe, when the believing women come to you as migrants, test their sincerity, although Allah knows best about their faith. Then if you find them to be believers, do not send them back to the disbelievers; they are not lawful [wives] for them, nor are they lawful [husbands] for them. But give the disbelievers whatever [dowries] they have spent[4]. And there is no blame on you if you marry them, as long as you give them their dowries. Do not hold on to the marriage ties with the disbelieving women[5]; ask for repayment of whatever [dowries] you have spent, and let the disbelievers ask for repayment of whatever [dowries] they have spent[6]. This is the ruling of Allah that He judges between you, and Allah is All-Knowing, All-Wise.

[4] i.e., compensate for their loss. [5] i.e., do not stop them from joining the disbelievers.
[6] If a disbelieving wife opted to join the disbelievers, a Muslim husband could demand in return the equivalent of her dowries. Similarly, the disbelievers had a similar right if a believing woman joined the Muslims. This and the following verses were revealed after the Treaty of al-Hudaybiyyah.

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.[1676] 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.[1677] That is the judgement of Allāh; He judges between you. And Allāh is Knowing and Wise.

[1676] For marriage, i.e., compensate their loss.
[1677] 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.

10. O you who believe! When believing women come to you as emigrants, examine them; Allâh knows best as to their Faith, then if you ascertain that they are true believers send them not back to the disbelievers. They are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them. But give them (the disbelievers) that (amount of money) which they have spent [as their Mahr][3] to them. And there will be no sin on you to marry them if you have paid their Mahr to them. Likewise hold not the disbelieving women as wives, and ask for (the return of) that which you have spent (as Mahr ) and let them (the disbelievers) ask back for that which they have spent. That is the Judgement of Allâh, He judges between you. And Allâh is All-Knowing, All-Wise.

[3] (V.60:10) Mahr: Bridal-money given by the husband to his wife at the time of wedding.

O ye who believe! When believing women come unto you as fugitives, examine them. Allah is Best Aware of their faith. Then, if ye know them for true believers, send them not back unto the disbelievers. They are not lawful for them (the disbelievers), nor are they (the disbelievers) lawful for them. And give them (the disbelievers) that which they have spent (upon them). And it is no sin for you to marry such women when ye have given them their dues. And hold not to the ties of disbelieving women; and ask for (the return of) that which ye have spent; and let them (the disbelievers) ask for that which they have spent. That is the judgment of Allah. He judgeth between you. Allah is Knower, Wise.