Dear Teodor,
You are right in using rf130-saldo but the issue is that this field is not computed in first page. you may have to modify standard program such that call to form ausgabe_anschreiben_text is postponed until the value is computed.
As workaround is the legal requirement that only one page should be printed? can you not print 2 pages and send the second one which has all content to your customer? Just a thought as I have no idea of your exact requirement.
Kind Regards
Soumya Rao