Is it permissible to accept an additional amount in one's salary that was intentionally added without the knowledge of the company owner, knowing that returning the amount might harm the person who calculated the salary?

If the employee agreed that his salary would be effective from a certain day, and the company owner did not fulfill his commitment and paid the salary starting from the following day, then if this change was made by mutual agreement, the employee must return the salary for the extra day. However, if the company owner changed the payment date without mutual agreement, there is no sin upon the employee for keeping the salary for that day.