Did you maybe try with function "Write-off Prepayment"?
http://www.hansamanuals.com/main/english/mailnumber___3396/theconf___516/manuals/levelzero___3393/version___71/hwconvindex.htm
The register for Order Classes is common for Sales orders and Service orders. When I set an Order Class to create an Activity - it works for Sales Order, while for Service Orders - not. Is any setting...
