What should I do to return a sum of money I took without its owner's knowledge, as I have repented, but I fear problems if he finds out?

You must take the initiative to return the stolen money to its owner along with sincere repentance. If direct restitution causes embarrassment, you can use a stratagem to return it by any possible means, such as sending it as a gift or a right due to him through another person. The important thing is to deliver it to him as soon as possible and in a way that does not cause a problem.