Exch. rate and currency key copied from purchase order &

What causes this issue?

A fixed currency exchange rate has been specified in the purchase
order. For this reason, posting can only be effected using the currency
and exchange rate stipulated in the purchase order.

How to fix this error?

If you receive this message on the initial screen:
The system has overwritten the currency and exchange rate with the data
stipulated in the purchase order. You cannot make any changes.
If you receive the message after <LS>New item</> --> <LS>Purchase order
</>:
Since you cannot change the exchange rate during processing, you
cannot select this purchase order for this document. You must terminate
the selection and process the invoice item for this purchase order
separately in another document.

Error message extract from SAP system. Copyright SAP SE.