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I'll share insight on how you can fix this situation with your bill and its payment in QuickBooks Online (QBO).
You can run the Unpaid Bills report to locate this certain amount that you still owe. Search for the bill in question, since the payment was possibly linked to a different bill.
If that's the case, follow the steps below:
At this point, open the correct bill transaction where the payment is supposed to be linked.
Otherwise, check if there's a duplicate or a different bill for that vendor. It would explain why you still have amount owed for that vendor.
I suggest you take a look at this article as well: What to do if you see unapplied cash bill payment expense on your profit and loss. It has the steps you can follow to search for bill payments that weren't applied to bills.
Feel free to tag me in the comments below if you need anything else. I'll be sure to get back to you.