If a custie said they paid by cheque and you have not received the amount is still outstanding and due unless they can prove by there bank statement as proof that it has gone through there account. That`s legal.
I once was robbed and lost about 10 large cheques and my attorney said the debt was still outstanding until they could prove I received that cheque and it was cleared by there bank. As a cheque is a promissory note to pay,and as such not a payment until in my account. If a receive cash I write a receipt as its just good business practice. Custies also see you are more professional and don`t seem to argue so persistently.