holding Backpay because of accounts receivable

I need help, i resigned last September as a medical representative. I followup my back pay but accdg my previous employer they will not process my backpay because i remit a post dated check from my regular client dated january 16,2013 and they need assurance that the check will be cleared once deposit but it is an old client and there's no whatsoever history of bounce check in the company. Am i still responsible for that even if i am not connected with company anymore? my point is, it is not a new account and they have the check already. One more thing, i have a client (drugstore) that close and did not pay their order but the company was aware with that because i ask their help when i have a hard time collecting the payment until it close and they are still asking me about that even if they are aware that the drugstore was close. The company holds my backpay because of this two account. Is that legal to hold my back pay under this circumstance? Thank you in advance..

reply please..