[May a trustee borrow from property in his hands?]

Answer: It is not permissible for a trustee to transact with the property in his hand from among the trusts in any manner whatsoever; that is the very nature of trusts.
It may be permissible for the trustee to transact with the trust if the owner authorizes the trustee, or if it is the trustee’s known custom to transact—such as with banks and money-changers—since the established custom is that they transact with what is deposited with them, and the owner knows that; placing it with them with knowledge of their custom counts as authorization to transact.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2