QB Issue Resolution:

Using pay bills you can apply a discount to the bill, or create a bill credit and apply that to the bill, also using pay bills.

Resolution for Issue 'Enterered A/Pay in prior year (now closed). Vendor did not complete all the work. How do I remove the balance that still shows unpaid?' available: Yes (Solved).
Source: Intuit Community forum.

Enterered A/Pay in prior year (now closed). Vendor did not complete all the work. How do I remove the balance that still shows unpaid?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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