All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ʻalayhi wa sallam, is His slave and Messenger.
We believe that there is no objection to saying ‘In Shaa’ Allah’ in response to someone's request to commit a sin with the intention to make that action conditional upon the will of Allah – not in the context of the sayer's resolve to commit the sin indicating his weak religious commitment and ill manners with Allah, the Exalted. This is because no action in the universe, whether it is a good deed or a sin, comes to pass except with the will of Allah.
You did well by resolving not to obey your boss and commit this haraam (unlawful) which he asked you to do, as there should be no obedience to a created being in what constitutes disobedience to the Creator. You should advise your boss and forbid him from doing what is unlawful if you believe that he would listen to you and you do not fear that he might harm you as a result.
The Prophet, sallallahu ʻalayhi wa sallam, said, “Whoever among you sees an evil and can change it with his hand, let him change it with his hand. If he cannot do that, then with his tongue (by speaking out); and if he cannot do that, then with his heart (by hating it and denouncing it internally), and that is the weakest of faith.” [Muslim]
Allah knows best.