Sale Backorders Workaround

So, I have 50 pieces of an item.
I have sale orders for 45 of them from 3 different clients waiting for some other products in order
to print their invoice and ship the item, but its 3 months from now.
A 4th calls and wants 20 pieces.

How would you keep track of the backorders, in order to know how much you should order from
our supplier?

What is your workaround/handling of this?
I saw that @lubos is working on it, but my boss asks, so…

If you are using goods receipts and delivery orders, you will see a column for quantity owned. This is the number of unobligated items you are able to sell. See https://www.manager.io/guides/11128. Note, however, that a sales order does not obligate inventory. Only a sales invoice will do that. So you may want to raise the invoice with a future due date and not send it until you are able to fulfill the entire order.

What would be the downside of using a delivery note with future date instead of an invoice?

Manager ignores future-dated transactions until the date arrives.

I don’t think that’s correct. Sure, when you are asking for a report as of certain date or for the certain period, then transactions beyond that date or period are ignored. But if you are viewing balances under any tab, current date is not considered. The balance will include future-dated transactions too.

If I understood @Giorgos_Arnakis correctly, he was asking about delivery orders, which wouldn’t affect account balances, only quantities. So, if entered with future dates, quantity today would be unaffected.