It is not allowed to change price from &1 to &2 for POB &3 Cond. Type &4

What causes this issue?

It is not allowed to change the sign of contractual price from positive
to negative and vice versa.

System Response

If the cost-based COPA is active, the SHKZG_VA value needs to be set for
each posting item that is related to revenue and cost based on the
contractual price of the original or contractual price.

How to fix this error?

This message can be customized in the message control Customizing
setting. The system responds depending on whether the status of this
message is set to Warning or Error or switched off.
Warning: This scenario is reported as a warning. Please be aware that
the system cannot automatically handle the inconsistent data. It is not
recommended to set a Warning if the cost-based COPA is active.
Error: The system will prevent the inconsistent data from being saved.
Therefore, processing an invoice RAI item or change price will fail.
Switched off: the system will ignore the check, this is only recommended
if the account-based COPA is active or COPA is not active at all.

Procedure for System Administrators

If the error occurs in your system, please check whether the RAI item
processing sequence is correct.
For example, the order is created with a price of 100 EUR, you issue an
invoice of 120 EUR, then reverse the invoice of 120 EUR.
If you process the reversed invoice RAI item earlier than the invoice
RAI item, the contractual price might be changed from 100 EUR to -120
EUR, which results in an error
For this scenario, please follow the business sequence and process the
RAI item in a strict order.

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.