Requsition change request is getting Rejected after Manager Approval.

We have one requisition change request which is getting REJECTED after Manager Approval. Upon Investigation, it is found that below,

select * from apps.po_change_requests where document_num='51703'

Here it is rejected and Reason is below :

Exception when processing the change request : Cannot update price.

Cause: You cannot update the price for one or more of the following reasons:

1. You have receipt accruals against one or more of the shipments  
2. You have matched invoices to one or more of the shipments.

Validation Error : 013:

1) Can you please let me know when this exception happens and what I need to check to confirm the same?