how to create a page composer to override a AR refund segment value before SLA rules?

Summary:

Only one receivables refund activity is allowed in AR. For regulatory LE´s, a segment must be informed.

For example, refund activity created for Legal Entity = ABC with the regulatory segment = ****.

When a refund is saved for a Legal Entity DEF (the correct regulatory segment = ZZZ) , show an error message informing the Code Combination is disable, once is generating using the regulatory segment = ****.

How to create a page composer, that based on the Legal Entity to update the regulatory segment for the correct value?

Content (please ensure you mask any confidential information):

There is no regulatory segment as "fake" to be used in receivables refund activity setup to be updated later using SLA rules.