Considerations for Approval Process: Field updates don’t evaluate custom validation rules on fields

Requirement

I came across one scenario where the requirement was to prevent an approval process field update action when a Credit/Debit record is in Invoiced status though the record is still in the approval process.

Background

When a Credit/Debit record is submitted for approval process, there may be scenario where the approver may be on leave or may delay in approving that request. In the meantime, SAP, an external integrated system, can make an update on this record by invoicing the data even without any approval notification from Salesforce.com to SAP.

Actions taken

I thought of keeping a validation rule to prevent the approver from making any change if the record is invoiced from SAP.

However this didn't happen because of this simple fact below.

Facts

One of the considerations for Approval Process is "Field updates don’t evaluate custom validation rules on fields". So in this case, a custom validation rule won’t fire inside an approval process.

To be continued..

The search for some other workaround is ON..