You are creting a Credit Memo and try to enter the Freight information, but it was already fufilled and grayed out.
SAP Business By Design.
Reproducing the Issue
Go to the Customer Invoicing work center.
Select the Invoice Documents view.
Search for the Invoice Document ABC (ABC represents the Invoice Document).
Click on the Follow Up button.
Select the Credit Memo option.
On the Credit Memo view, note that the Freight field is fufilled already and grayed out.
When you create the Credit Memo from the Invoice ABC the system will pick up the Freight information from the Invoice ABC and it won't allow you to change this information.
It is the expected system behavior.
Credit Memo ; Freight ; Invoice , KBA , SRD-CRM-INV , Customer Invoicing , How To