Exchange rate and currency copied from contract &1

What causes this issue?

A fixed exchange rate has been defined in the entered contract. For this
reason, you can only use the currency and exchange rate agreed in the
contract when posting.

System Response

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

How to fix this error?

If you see this message on the initial screen:
The system has overwritten the currency and exchange rate with the
agreed currency and exchange rate from the contract. You cannot make
changes.
If you see this message after choosing <LS>New Item</> -> <LS>Contract
</>:
Since you can no longer change the exchange rate during processing, this
contract cannot be selected for this document. You must cancel selection
and process the invoice item for this contract separately in its own
document.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.