FMDERIVE For Asset Transfer

Dear FM Gurus,

I am facing an issue in fmderive when execute transaction ABUMN, in the system we want no budget checking/availability control when we execute ABUMN. so, we want to create derivation rule assignment specifically for this transaction. But, how do we distinguish ABUMN with another fi posting transaction?because only Business Transaction RFBU appears in FMDERIVE tracing as shown below :

When we try to refers Ref_Transaction AMBU as source field we got error like this below:

am i missing step here? or my approach is wrong?

Regards,

Angga