Is it necessary to ascertain the religion of my maternal uncle's grandson (his son's son) before sending him the financial amount bequeathed by my grandfather, especially since he was born and lived in America and it is unknown whether he is Muslim or Christian?

The basic principle is that a person is considered Muslim as long as they are born to a Muslim father. Their disbelief is not ruled upon unless it is proven by their confession or by the testimony of two just witnesses. The witnesses must detail the specific aspects of their disbelief.

If you are certain of his Islam and that he is not an heir to his grandfather, then the will made in his favor is valid, and he should be given the full amount if it does not exceed one-third of the estate. If it exceeds one-third, the excess portion is not executed unless it is approved by the adult heirs. Similarly, if he is an heir, the will is not executed except with the permission of the heirs.

If his apostasy is proven, jurists have differed regarding the validity of a will made for a specifically named apostate. The majority believe it is invalid, while the Shafi'is and Hanbalis, in the more sound opinion, consider it valid for a named individual, as long as he does not join a hostile land and become inaccessible to Muslims, or die while remaining an apostate.

Since this issue is contentious and apostasy is not proven except under specific conditions, it is necessary to refer the matter to the Sharia court for a ruling.