QB Issue Resolution:

If you want to see Open AR Payments as AR Credits, you run AR Aging Summary (or Detail) and Filter on Amount <= 0. Please see my attachment. I use this for Customers With Credits Available.

Resolution for Issue 'How do I find payments where full amount was not applied to invoice?' available: Yes (Solved).
Source: Intuit Community forum.

How do I find payments where full amount was not applied to invoice?: 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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