What is the ruling of Islamic law regarding the reservation of a commodity for a person, equivalent to half the money he promised but did not fulfill his promise, and this is done without his knowledge?

A person is obligated to fulfill their promise of a loan, especially if the promise led the recipient to incur a cost, which has been affirmed by the Islamic Fiqh Academy. If he does not abide by the promise, then there is no blame upon you for taking the equivalent of the loan from his money without his knowledge to avert harm. Thereafter, you must inform him of the matter, clarify that you relied on a legal fatwa (definition), and write a document for him detailing what you have taken.