Retained amount for reversal is set automatically for full reversal

What causes this issue?

The quantity for which you are entering a credit memo is the same as the
quantity calculated to date. You are therefore performing a full
reversal of the quantity. This means that the full amount of the
retention must also be reversed.
If the purchase order is for a service, the calculated retention for
services is reversed with the credit memo. For this reason the system
only allows the full amount of the retention for the credit memo.

System Response

The system calculates the full retention and overwrites the entry in the
<LS>Retention Amount in Document Currency</> field.

How to fix this error?

Check the retention amount. You may need to flag the credit memo as a
<DS:GLOS.138E06E63DF2D211BDEE0004ACE6148F>subsequent debit</>, so that
the calculated quantity or the calculated service remains unchanged.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.