What is the ruling on money earned from staying an extra day after completing a task in 3 days instead of 4, knowing that the contract does not stipulate completing the four days to receive the financial compensation?

What determines whether you can keep the money is the contract made between you and your employer:

1. If the allowance is given to the employee regardless of their expenses, and they are not required to return the remainder, then there is no sin in you benefiting from what is left with you. 2. However, if the employer gives the employee only their expenses and needs, and any excess must be returned, then it is not permissible for you to take the remainder, because the money was not given to you for ownership, but rather you only own what you have used for work purposes.

You can refer to your employer to inquire about the nature of this money.