The primary possibility here is that GB may have indicated a sales tax owed by the buyer. But, that amount might not have been the actual tax rate needed to be collected.
I used to have to fight Quickbooks on this, as QB would assign a tax rate of 5%, and at the time, the Texas rate was 7% (varying by locality). Which was even more frustrating for my tax exempt sales, to justify the invoices to "credit back" the 5% taken out.
If GB "under-billed" the tax in the original transaction, they'd be required to make a second demand to collect the rest (Kay Bailey Hutchinson's minions were always sending me adjustment dunning--and credits, too--it was part of having a Texas Sales Tax Certificate and doing business in Texas). Paying sales taxes in Louisiana was equally complicated.