Next change to main item redetermines the free goods quantity

What causes this issue?

You have changed the quantity in the free goods item manually. If you
change the quantity or the pricing date in the main item, the system
redetermines the free goods quantity according to the master data.

System Response

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

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.