Reverse entry value set automatically for full reversal

What causes this issue?

The quantity for which you are entering the credit memo is the same as
the quantity invoiced to date. You are carrying out a full reversal on
a quantity basis. In this case, the total invoiced value must also be
reversed - otherwise you could be left with a value but no stock for
this material.
If the purchase order was for a service, the service invoiced will be
cancelled when the credit memo is posted. Therefore the system only
allows the total value to be entered in the credit memo.

System Response

The system will calculate the total invoiced value and overwrite the
entry in the field <LS>Amount</>.

How to fix this error?

Check the amount. You may have to post the credit memo as a subsequent
credit, with the quantity or service invoiced remaining the same.

Error message extract from SAP system. Copyright SAP SE.