Praise be to Allaah.
If he has indeed divorced her by khula’, in the sense that
the marriage has been annulled and there is nothing left to be done except
handing over the compensation (i.e., the mahr), then he has no choice in the
matter, even if he has not yet taken back the mahr. But if they have agreed
to khula’ without yet having the marriage annulled, rather they have agreed
that he will let her go when she hands over the mahr, then this does not
mean that the marriage has been annulled, rather it is promise to annul it.
So if it has not yet been annulled, then he has the right to change his mind
and not do that which he had intended. If he had said, “If you give me the
mahr you will be free (I will give you khula’),” then according to the
Hanbali madhhab he does not have the right to change his mind. But according
to Shaykh al-Islam Ibn Taymiyah, if he has not yet taken back the mahr then
he has the right to change his mind. In order to be on the safe side, if the
latter scenario has taken place and they want to get back together, they
should make a new marriage contract so as to put themselves beyond any area
of scholarly dispute.
Fataawa al-Mar’ah al-Muslimah, 2/785; fatwa of Shaykh ‘Abd al-Rahmaan al-Sa’di.