QB Issue Resolution:

You may prevent QB from automatically applying payments to invoice transactions by unchecking the option Automatically apply payments from your Preferences.

Edit menu> Preferences> Payments> Company Preferences> Automatically apply payments

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Resolution for Issue 'Enterprise Solutions 15.0. Payment defaults to oldest open invoice, no way to change it?' available: Yes (Solved).
Source: Intuit Community forum.

Enterprise Solutions 15.0. Payment defaults to oldest open invoice, no way to change it?: 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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