Why doesn't the balance of my unpaid bills detail match my balance on the vendor detail in quickbooks?

Hi there, @GLA. Happy to provide some insight on how these two reports calculate the balances.

Both the Unpaid Bills Detail and the Vendor Balance Detail report utilize the same information to calculate balances. If you notice a balance isn't matching for a certain Vendor, it can be caused if a journal entry was manually entered and affected Accounts Payable for that Vendor. I recommend checking to ensure that's what happened in this instance. I also recommend checking the dates on the reports to ensure they both gather information from the same period.

If you'd like to make customizations to Vendor reports, I recommend checking out: Customize vendor reports

Please reach out if there's anything else I can do to help, I'm determined to be your number one resource for QuickBooks Desktop. 

I filed GST and now it won't let me pay it.  The bill keeps appearing in the pay bills window.  This is making my A/P balance wrong but the Vendor Balance is correct.

Thanks for jumping in, Charlotte21.

There could be a journal, a vendor credit, an adjustment, or a payment that offsets your vendor's balance that is not linked to the bill that's why the bill still shows up when you make payments. Let's check it out.

For example from the same vendor, I have an open bill amounting $10 and a vendor credit amounting $10 (unapplied). When I look at the vendor balance detail report, it shows 0.00, but when I make pay bill transactions, I see the $10 bill waiting to be paid. This is because they are not linked together.

To check which journal, credit, adjustment, or payment is unapplied for that particular vendor of yours, you can make a pay bill transaction. Then, click on Set credits at the bottom part. This will show you what transaction it is. Then, try to remember why is it not applied to any bills, so you'll know what to do with it.

By the way, you can check about paying sales tax in QuickBooks Desktop.

We're just right here if you need further assistance with your tax or vendor balance. Have a good one!