How can one atone for stealing money from former employers when the exact amounts are unknown?

Taking people's money unjustly is a grave sin. The perpetrator must repent sincerely and return the rights to their rightful owners. Search for the owners of the money or their heirs to return it to them, even if indirectly. If it is impossible to reach them, then donate it to charitable causes. If you are unaware of the amount, act upon what you most likely believe it to be, while exercising caution.