In PO &, currency & and exchange rate & are fixed

What causes this issue?

In purchase order &V1&, currency &V2& and exchange rate &V3& were
fixed.
Currency &V4& was entered in the invoice document and differs from the
fixed entries.

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 message appears when processing invoices online, you must
correct the invoice amount and the currency identification code.
To do this, enter currency key &V2&.
You can also delete the currency key in the initial screen. In this
case, the fixed currency key and exchange rate are automatically taken
from the purchase order.
If this message appears when processing invoices in the background, you
must delete the invoice and re-enter it with the correct currency key.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.