Is giving in charity an amount mistakenly overpaid in one's salary sufficient, or must it be returned?

The money must be returned to its owners. Donating it on their behalf is not sufficient as long as you are able to return it. You are not required to inform them of what happened; rather, it is sufficient to return it to them by any means that does not cause harm or embarrassment. It is permissible to use stratagems to return the money to the workplace.