buyer threatening to cancel post dated cheque post sale agreement

I did a sale agreement for my play school last week. I insisted the buyer to check all the materials and premise at the time of handover. There were no issues then. But after a week, the buyer has sent me a mail that there are items missing, which is completely wrong. He's threatening if I don't compensate, he will cancel the post dated cheque that he has issued me as part of sale agreement. How should I deal with the situation?