QB Issue Resolution:

Set up a Product/Service called Retention.

For the Income Account add an account to store the Retention in (I would make it a current asset account called Retentions)

When you create an invoice add Retention as the last line and put Retention in as negative.

Resolution for Issue 'I’m sending out a client invoice and need to hold retention, how do I do this?' available: Yes (Solved).
Source: Intuit Community forum.

I’m sending out a client invoice and need to hold retention, how do I do this?: 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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