fryshaun said:

 Is there a way to modify existing records using this format?  

No, importing customers will fail on each line for an existing customer.  To actually modify the record you would have to either:

1.  Use the Sage 50 SDK to locate and update the customer record, (strongly preferred!) or

2.  Use an SQL query to directly update the customer record in the tCustomr table.

fryshaun said:

Currently the customer record is denied because it already exists.

The .IMP format can add new customers, but appears to be designed to not touch existing 'ledger' records (customers, accounts, inventory items, etc.)

fryshaun said:

POS system modifies invoice to reflect a full or partial refund.  This information is then uploaded to accounting.

The Sage 50 SDK appears to have methods to reverse or adjust an existing invoice. 

AFAIK To do this with .IMP text files, you have to generate a new, return invoice, and post as a negative invoice, referencing the first invoice. 

The Sage 50 Canadian SDK can be programmed in .NET, and comes with sample C++, C#, and VB code.   It has been available for free, for about 6 or 7 years.  The text .IMP import has been available as far back as I've used Sage 50, and seems to be in maintenance-only mode since around 2007.

I hope that helps, please post back or send (either way) and mark the any answers you find helpful.