Hi,
The challenge with using F.13 and assignment field for clearing customer invoices with bank statement items is, that you first need to find out who the customer on the bank statement is. If you are not able to post the bank statement item to the proper customer account then you cannot clear it. Also, some customers pay multiple invoices together with one transfer - and then F.13 is out of question because the bank statement item cannot have two different assignments at the same time.
Normally, you perform the clearing through bank statement upload itself (trans. FF_5 or FEBP):
- Configure a 2-step posting rule (area 1: Debit Bank - Credit Cash in transit; area 2: Debit Cash in transit - clear subledger credit account)
- When assigning posting rule to external transaction code, choose the proper interpretation algorithm. The most useful ones are 020 - search by document number or 021 - search by reference. It means that when processing the bank statement, the program will search for documents with either the reference or the document type - across all customer (or vendor) open items - and if there is a match with the same amount then it will make a clearing. These two fields should be used for searching if possible. There is also a search string for assignment but it does not find the customer by itself - therefore it is not very useful.
- In the bank statement posting program variant (trans. FF_5 or FEBP), define a reasonable range of either document numbers or references that customers may use as invoice numbers.
So, if you e.g. assign a posting rule to an external trans. key with interpretation algorithm 021 (Reference), enter a range of references in FF_5 e.g. 6000000000 - 6099999999 and a customer sends you a note to payee e.g. "JOHN DOE INDUSTRIES INV 6056787656 250.67 USD INV 6056787658 15.06 USD" then the program will search for customer and vendor open items with references = 6056787656 and 6056787658, and if their total matches with the payment item amount (within tolerance, if applicable) then it automatically clears them. This is the usual way of clearing incoming payments. F.13 has no use here.
This is a big topic but I cannot write more now. You may check similar questions e.g. http://scn.sap.com/thread/3445188 or http://scn.sap.com/thread/3444644 . Try this, and let us know if you still have any issue. Good luck!