Hi Harshal, thanks for your input. I can join the data source, but I can't use the existing key figure 'Cash Ledger Open Item' in Cash Open Items report. The key figure is SAP designed calculated, I have no clue how it's calculated. And I am not sure what characteristic the 'key date' the cash open items report is referring to?
But anyway Checkbook Register Report works, we just have to print two reports.