What is the ruling of Islamic law on disposing of entrusted money without the owner's knowledge due to hardship, then returning it without their knowledge after the situation becomes easier?

It is obligatory upon whoever holds a trust to preserve it and not to dispose of it except with the owner's permission. Whoever disposes of it without the owner's permission is sinful and is liable for compensation if it is lost after his disposal. What you did by disposing of the trust without permission is a grave mistake. You must seek forgiveness from its owner, unless you fear a greater harm, in which case you should supplicate and seek forgiveness for him. If the disposal resulted in a profit, it must be returned to the owner, unless he permits you to take it.