How can we reject a Sales order after first Down payment invoice for Italy?

Hello,

As a legal requirement in Italy the Down paymenty have to be transformed into real invoices, another legal requirement is that an invoice cannot be cancelled it has to reversed by a credit memo.

I would like to know in this context how we can reject a sales order line once a Down payment invoice has been emitted.

When I do enter a reason for rejection in the sales order line I receive the message : V1036: Subsequent documenst exist for item XXX and then V2432 Cancellation not possible. Down payment request already created.

I can create a down payment credit memo in order to cancel the down payment invoice but how can I get rid of my SD requirement in MD04 if I cannot reject the sales order line?