Unfortunately, the sales order form does not have a functionality to allocate header discounts to the different invoice lines.
So, you would have to setup the discount granted at the line level.
Regarding the special posting you want to have against ‘sales - promotions’, setup ledger special ledger accounts for the discount posting under inventory management/setup/posting/posting sales order tab, discount section.
I’ve found the “Total discount %” under the price/discount tab of the SalesTable and I’ve tried that. I would think that if I applied 20% discount, it would apply equally to the lines and during posting, each line would post 20% less to their respective accounts. This is odd to me.
I feel silly for not noticing that before, but after trying the header-level total discount, it still posts the $1000 to sales, the $200 to deferred revenue, and then the $240 to “Cash/Invoice discounts”. Our accountant is complaining because she wants the deferred revenue reduced appropriately instead of the discounts all being applied to one bucket if that makes sense.