Is it permissible, in Islamic law, for a person to take a sum of money given to him as charity to be distributed among the poor, as a loan for himself, with the intention of repaying this loan to the poor when he has the money available?

If the giver specifies beneficiaries for the money, and you are not among them, then it is not permissible for you to take it. You must return it to them and repent. However, if you are among those specified by the giver (for example, if you are poor and he ordered it to be given to the poor), then it is permissible for you to take your share and you are not required to return it. It is not permissible to borrow from this money except with the permission of its owners. If you do so, repentance is obligatory, and the money must be returned immediately.