EBS posting on customer account

Hi, sometimes I have payments from customer that I do not know which AR invoices to clear yet so I would like to book the payment on customer account without clearing. My question is how SAP would identify the customer number? In the incoming EBS from bank, I have customer name and IBAN number. I know I can use search string but that would require to do search string for each and every customer so it's not an option.

Anybody has any suggestions? The posting rule I want is like below:

Posting area 2: posting key 40 debit bank; posting key 15 credit customer