do we know how the credits got on the account? if it was added to the update customer box, there is no way of tracking it.
are these current credits, or have they already been applied to an invoice?
if a credit has been posted and either applied to an invoice or still hanging they will show up on your "balance due statement with credit invoices" report. this report is exactly what it says. it will show anyone with a balance due and that had a credit invoice posted to the account.
Instead use "balance due statement dbl vue 9" report. this report will give you a statement for anyone with a balance (credit or due).