Is it truly right to be obligated to falsify the number of workers by registering 20 workers instead of 15, knowing that the client is aware of the shortage, and despite my fruitless attempts to advise my manager?

What your boss does in terms of forgery and cheating with money is forbidden by Islamic law, and it falls under the saying of the Prophet, peace and blessings be upon him: "Whoever cheats us is not of us," and his saying, peace and blessings be upon him, when asked: "Can a believer be a liar?" He said: "No." Your participation in this act or your silence about it makes you a partner in sin. Therefore, your frankness with the client about the truth is correct, and it is obligatory for you to do so, just as it is obligatory for you to leave the job if your boss does not stop cheating and forging and you do not find a way to prevent him.