Is the financial bonus offered by a private office to government employees after the completion of a work project considered permissible (halal) or forbidden (haram), knowing that the office will pay the department all financial requirements?

It is not permissible to accept this bonus, as it is a means to bribery. The norm is that the office owner does not pay it except to incline employees to serve his interests at the expense of the public interest. The Prophet, peace and blessings be upon him, "cursed the one who gives a bribe and the one who takes a bribe."