Is it permissible for a husband and wife to transfer ownership of an apartment to an orphaned girl they have sponsored, knowing that the apartment is rented from the state and can be owned for a sum of money, and would transferring its ownership to the girl legally prevent their heirs from inheriting it?

If a husband and wife own an apartment, they have the right to dispose of it in a way that is permissible by Islamic law, such as selling, gifting, leasing, and so forth. If they wish to gift it to this girl while they are in good health, the gift is valid, and the girl takes ownership of it, and the heirs have no share in it. However, they are not allowed to intend thereby to deprive the heirs. A gift is only owned through complete possession, meaning the giver relinquishes control and the donee disposes of it as an owner.