QB Issue Resolution:

No, you not modify or edit and change an invoice

Use a non inventory item for the discount which posts to a sales income discount account

use that item on credit memo, enter the amount of the discount,save, and then apply the credit memo to the invoice

Resolution for Issue 'receipt a payment that has a settlement discount' available: Yes (Solved).
Source: Intuit Community forum.

receipt a payment that has a settlement discount: 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.

Support for Invoice
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Error Recovery for Invoice [Canada]
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