Yes, it was entered as a voucher in A/P-B.  It was not a purchase order or a commission.  I have tried to do the look up on it, but it shows no invoices for this vendor yet it shows up on the vendor aging and also under A/P-F (Pick vouchers to pay).  Normally I can back out an invoice or reverse it, but for some reason this one just keeps saying that invoice number already exists.  Any ideas?

Use Maintain Database on the BKAPINVT and BKAPCHKH files and look for the matching entries, or brute force, find the entry based on vendor and invoice date in BKAPINVT and just delete it if its 15 years old.

There is a utility program FIXAPAGE.RUN that should help but once it is run, there may be a few stragglers that will need to be edited in Maintain Database.  The latest version of the utility is available at www.istechsupport.com/dl/fixapage.zip; unzip to DBAMFG and run FIXAPAGE at UT-A

Once it is run, to clean up any stragglers, you need to use Maintain Database and find the invoices in the BKAPINVT file and make note of the "Invoice Number" which is actually a reference number and then open the BKAPCHKH file and find the payment(s) associated with the invoice (based on vendor code and invoice date) and edit the Invoice number to match the entry in BKAPINVT so the Aging program can match them up.  Or, if they are extremely old entries and you don't care about the history that far back, you can delete the entry from BAKPINVT.  

The AP-I and AP-F programs use different logic.  AP-F looks at the AMTRM field (amount remaining) in BKAPNIVT (invoice file) while AP-I starts with the original amount in BKAPINVT and then subtracts based on payments in BKAPCHKH (payment file) up through the "as of" date of the aging.  Sounds like you have BKAPCHKH records that aren't linking right with the original invoices, causing paid invoices to show on the aging with a balance due even though the amount remaining is actually 0.  Take a look at both files in Maintain Database, email me if you need help sorting it out.