Partial payment tab problem when clearing two open items in vendor account
I have the problem with partial payment procedure.
As SAP is presenting the process, partial payment should work as follows (i.e. with vendor):
1. Incoming invoice is posted -> Credit -100
2. Credit note is posted -> Debit +20
3. Payment (without clearing) is posted to vendor account -> Debit +50
I'll run transaction F-44 and go to tab "Partial payment".
I see 3 items (as above).
I mark Item -100 and +20
I double click in filed next to -100 and it changes to -20
An now it should make no posting but only assignemet of one item to another.
But in my case, if I click "save" icon, it makes postings Debit 20 and Credit -20 on vendor account.
Is there any possibility that such clearing will not generate any posting but just make assignement of one item to another. Or maybe there is something wrongly configured in the system. At the moment the partial payment makes some kind of residual items.
Thanks in advance for replay.