Is it permissible for a recipient to borrow a portion of the money designated for the needy to pay off his debt, with the intention of returning it and donating it later, knowing that he did not inform the donors of his need?

It is not permissible to borrow from money entrusted for distribution to the needy, because the agency was for charity, not for lending. Regarding taking a portion of it for oneself when in need, there is a difference of opinion among scholars. The more cautious approach is to inform the principals of the need for the money, and if they grant permission, then there is no harm.