You have nothing to deduct here. Borrowed money is not your money. It never was, is not now, and never will be. Therefore you never paid taxes on that "borrowed money", so you can't deduct it when you pay it back. If you are being charged interest you can't deduct that either, because you can only deduct interest on a secured loan (such as a home mortgage) or qualified Student Loan Interest.
While what your boss is doing is legal, it's unethical. But there's nothing you can do about it other than find another job and quit. Of course, that could result in your employer taking legal action against you. Doing so would cost them money, and if there is not any type of formal loan or payback agreement, a good lawyer representing you could easily "turn the tables" on what I see as an unethical employer.
But I digress. I am not a lawyer, or even a "tax expert" by any stretch of the imagination. It's also outside the bounds of this user to user support forum to give legal advice. All I can do is tell you that tax wise, based on the information provided as best as I can interpret it, you have nothing to claim or deduct here. So for you, seeking legal advice in your local jurisdiction from a licensed lawyer may be in your best interest.
