Is it necessary to inform a friend that a sum of money was taken without his knowledge, with the intention of returning it, or is it sufficient to just return the amount?

You must return everything you took. If you do not know the exact amount, then exert effort and be cautious to return what will absolve you. You are not obligated to inform him of the matter, especially if it would lead to harm. You must return the 55 dinars that are certain, and refrain from doing so in the future, and seek forgiveness for what has passed.