Am I liable to my employer for a clients bounced check?

I worked for a veterinarian in my area for a short time. At the end of August a client came in, asked us to hold her check for services rendered unto her, for a few days. Since this was explained to me as something the Dr. allowed, I accepted the check, with no date written, as instructed. Subsequently, the office manager ran the check through a check writers machine and found this client has a history of bad checks. The Dr. chose not to deposit this check for fear it would bounce, and began to tell me everyday for the remainder of my employment that I owed him for the check. I have since quit, for this and other reasons. He has since sent me my last pay check through the mail, and included a letter that " I should stick to my moral duty and pay for the check..." Am I liable, should I ignore his plea for payment? Please help.