WOW I have forgotten more that I remember about version 6. I am going to make a couple of assumptions.
1 the files are btrieve
2 you have the ddf files
3 the A/P open purge has not been run
4 you have the skills to write the report
If these are true there is a table named apopnfil or apopnfil_sql. This table contains a record for every A/P transaction, vouchers, payments and credit memos. There is a transaction date for each record., depending on how you did credit memos there should be an apply to record that will link the records together. You may have to deal with the beginning balance issue. That depends on how the data was originally entered.
This should get you started.
Steve Henley
Trianglepartners.com
Exact Software consulting, sales and implementations.
If the only tool you can use is a hammer then all your problems look like nails.