Dear …
RE: __(Reference Name – Account/Invoice #)__
We are in receipt of your post-dated check #_____ in the amount of __($amount)__, dated __(date)__. I would like to bring to your attention, that __(your account with Name of Co./this invoice/etc.)__ has already been in arrears for over __(#)__ days. It is not acceptable that you would presume you can now send a payment, posted-dated for an additional __(#)__ days. Particularly, since you had already agreed that you would make a payment of __($amount)__ by __(date)__. Your attempt to pay this __(account/invoice)__ with a check dated __(# days in the future/date/ in _year_/etc.)__ is unacceptable to us.
Apparently, your word is not worth much, __(Name)__, as you do not follow through on your commitments. What’s even more disconcerting is that you are now also refusing to return my phone calls. Needless to say, all this is having a seriously big impact on the business relationship that we have had with __(you or name of their Organization)__.
Ignoring this situation further will only make things worse, __(Name)__. I suggest that you contact me as quickly as possible so that we can try to resolve this matter without further discord. As our business relationship is at stake here, I hope to hear from you soon.
Yours truly,