Buttermort by Jim Butterfield for the Commodore VIC20 - Thank you Bruce Thomas for tracking down this program!  Although "Buttermort" isn't the official name, this is the name I found it under when I was reading the 1983 Commodore Computer Club newsletter where I ran across it.  Bruce found this program on a .D64 disk image in a TPUG (Toronto Pet Users Group) CD compilation under the name "MORTGAGE.Z".