This is an issue that others as well as myself have brought up. Hopefully they'll address this soon as more people bring it up.
Right now the workaround as I do it is to ring up the sale (not using the invoice) as if it is happening now and then you can do a split payment. You'll have to go to the PC dashboard and mark the sale paid by Square (if they used a credit card). You can't mark it 'paid by Square' on the POS for some reason. You have to do it this way so the invoice and the sale aren't duplicated in your day end totals. It's clumsy for sure.
