How can I return the money I stole from my customers six years ago, the amount of which I do not know, while desiring to conceal my identity? Is it sufficient to estimate the amount, double it, and secretly place it in a mosque with a letter asking the officials to use the money for the benefit of the country?

One of the conditions of repentance is to return rights to their owners. Therefore, you must return the stolen money to its owners. If you do not know the exact amount, then exert effort in estimating it and return what you believe most likely clears your conscience. Concealing your past is desirable, and you can return the money indirectly. It is not permissible to give the money away as charity unless its owners cannot be identified. If the owner is later found, they are given the choice between having the money returned to them or accepting it as charity. You should also frequently seek forgiveness and perform good deeds.