Unable to change purchase agreement field in PR as PR creator

Hi all,

When I create PR, the field purchase agreement in the line details is automatically generated when I don't fill or delete the vendor information in the PR line item. I am not really sure how AX propose the purchase agreement number. (It might come from the procurement category, vendor account, etc.) But this field is grey out when I open as PR creator.

However this field will be changed into modifiable when PR is open by PR approver (when he/she receive the PR from workflow).

My questions are:

1. Why AX restrict PR creator to change the Purchase Agreement value?

2. How to make PR creator to be able to select the Purchase Agreement?

Thanks in Advance,

M.H.