Currency &2 and exchange rate &3 were fixed in purchase order &1

What causes this issue?

The currency &V2& and exchange rate &V3& were fixed in the purchase
order &V1&.
The currency &V4& that you have entered of the invoice document differs
from these fixed values.

System Response

Further processing of the invoice document is only possible if the fixed
currency key matches the currency key entered.

How to fix this error?

If this check was executed in online processing, you must correct the
invoice amount and the currency key.
Enter the currency key &V2&.
You can also delete the currency key on the entry screen. In this case,
the system will automatically use the currency key and exchange rate
fixed in the purchase order.
If this message was generated during a check in background processing,
you must delete the invoice and enter the correct currency key again.

Error message extract from SAP system. Copyright SAP SE.