Making currency as readonly field on requisitions page and issue being faced

Summary:

Hi Experts,

Client has a requirement to make the currency field as read-only on requisitions page. Supplier and site being mandatory on requisitions page , so currency is derived as per site selected. We have made the field as read only through sandbox, all works fine but issue comes when user is changing the item type on already filled requisition form with supplier site and associated currency. As soon as the item type is changed, currency code resets to functional currency of BU. Attaching screenshots to depict the case.

Is this behavior by design? How to overcome this issue.?

Content (required):