Hi Ferdinando,
You can't register a document in MIRO with 0 amount, at least you need 1 cent in the vendor to create the document.
I am confused with
1) F-48 for downpayment
2) FB60 for down payment invoice
What posting did you get in these steps
1) 29 A Vendor
50 Bank subaccount
2) ????
Kind Regards