The G/L account is not transferred from earmarked funds. (Down payment)

What causes this issue?

You create a down payment with reference to earmarked funds. In
Customizing for transferring the account assignment from earmarked f
unds, it is specified for down payments that the G/L account is to be
transferred.
This makes no sense because the reconciliation account or the
alternative reconciliation account used for down payments by the credi
tor or debtor is already used as the G/L account and a change could lead
to incorrect postings.

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

System Response

In down payments, the transfer of the account assignment does not
transfer the G/L account to the document if the G/L account (here, with
the reconciliation account) has already been filled.
In Customizing for down payments, create a user-defined rule for
transferring account assignments and assign it to transaction AZBU (Down
Payment). Deactivate the "G/L Account" rule step. Then you will no
longer receive this message.
Customizing activity Define Rule:
View V_FMRACCMVCL (<DS:TRAN.SM34>Execute Function</>)
Customizing activity Assign Rule:
View V_FMRACCMVASSIGN; (<DS:TRAN.SM30>Execute Function</>)
Alternatively, you can also deactivate this message.
INCLUDE 'CUSTOMIZED_MESSAGE_INFO' OBJECT DOKU ID TX

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.