What is the ruling on postponing childbearing with a Christian wife from the People of the Book, fearing her escape after childbirth or due to economic circumstances?

The Shafi'is held the view that coitus interruptus (al-azl) is permissible with a free wife unconditionally, based on Jabir's hadith: "We used to practice coitus interruptus during the time of the Messenger of Allah, peace and blessings be upon him, and when that reached the Prophet of Allah, peace and blessings be upon him, he did not forbid us."

However, the majority of jurists stipulated the permission of the free wife for coitus interruptus, unless there was a need for it, such as her being in a land of war and there being a fear that the child might apostatize. Accordingly, your brother has the right to abstain from procreating with his Christian wife if he fears for his offspring from her or from the country in which he lives. As for citing the economic situation as a reason, it is not permissible, because Allah has guaranteed sustenance for His servants.