Is it permissible for someone who has taken money without its owner's knowledge to return it without informing him, and is it permissible to settle the original owner's obligations with this money with the intention of returning it?

Whoever takes the right of another must repent and show remorse, and return the right to its owner by any possible means. It is not permissible to fulfill the obligations of the right-holder with the intention of returning the money to him, because what is required is to return the right and transfer its ownership to its owner.