difference in clearing documents

Hi,

I would like to ask for help with the following problem. When posting the clearing (eg. clearing of Vendor Invoices X Bank payments on vendor account) the clearing documents are posted in two ways:

Example clearing of two documents (invoice and payment) in amount of 100 in document currency.

In the first way the clearing document is posted in document currency:

DR: 100

CR: 100

In the second way the clearing document is posted in document currency:

DR: 0

CR: 0

I am not able to find out how sap decides in which way to post clearing document.

I checked the cleared documents and never found out any difference which could cause the way of posting of the clearing document.

Thank you,

Jan