Can my employer deduct money from my pay for a bill that my husband acquired before we were married?

I work for a privately owned family dentist office. I have worked there for over 3 years. While my husband and I were just dating my boss performed some dental work on him and the bill was never paid due to us having some financial troubles. My last check was $50 short and a note was attached to the bottom of the stub that said it had been deducted for my husbands previous bill. I do not make a lot of money and my husband and I are still having financial problems. Can they do this legally? What should I do?