Is it necessary to inform a colleague when returning money taken from him without his knowledge, or is it sufficient to return it without informing him? And if the amount is not remembered, should the lesser or the greater amount be returned?

Money taken unjustly must be returned to its owner or by seeking their forgiveness. It is permissible to return the money without the owner knowing that you were not entitled to it, in order to avoid harm. For example, you could send the money to them with a trustworthy person, who would inform them that this money is from someone who took it from them and has repented. If you are unsure about the amount, take precautions and return the larger sum.