Is it permissible for an employee who has a sum of money allocated for purchasing alcohol and gifts to use it for himself if he is in need, with the intention of donating it later, knowing that returning the money would lead to its use for the same prohibited purposes?

The money remaining from the amount spent on purchasing alcohol must be returned to the governmental entity that disbursed it if that entity will spend it on permissible matters. Otherwise, it should not be returned to them, and the questioner should spend it on general Muslim welfare. If the questioner spent the money on themselves, then if they are poor and in need, nothing is required of them. Otherwise, they must give an equivalent amount from their own money and spend it on Muslim welfare, and the poor person is permitted to take from it what suffices their needs.