Praise be to Allah.
We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah preserve him), who answered as follows:
Yes, you are still in debt to them, because when they did not cash the cheque, the money stayed in your account at the bank. You are in debt to them because the check is merely an instruction to the bank to transfer the money. And Allaah knows best.