Is it permissible for an official in charge of a foundation's funds to borrow from them for his personal needs, provided that he commits to repaying them when repayment becomes possible or immediately when the foundation needs them, and does this affect trustworthiness?

The funds in your possession are a trust (amanah), and you are a representative of their owners in disposing of them within the specified parameters. It is not permissible for you to borrow from them or lend from them except with the permission of their owners. Your commitment to repaying them does not justify such actions, because your possession of them is that of a trustee (yad amanah), and disposal of them must be in accordance with what has been explicitly or customarily authorized for you. Any other disposition is considered a betrayal of the trust, and Allah Almighty says: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]." And the Prophet, peace be upon him, considered betraying a trust to be one of the characteristics of hypocrites.