This sounds great in theory - the way Square and Quickbooks deal with this is infuriating; it should just be an option in the reconcile window, where you select add/transfer etc.

Two things: one, if this is an immediate transaction, great, I will start recording them as sales receipts...however, I won't know the exact amount until I get my square payment, will I? What about rounding etc.

Two: can i go back and edit former transactions to turn them into sales receipts? I don't really want two forms of sales; I write invoices. Some may be paid immediately, some may not. I might email an invoice off to someone following, say, a remote transaction (remote computer support) and get a call back in 15 minutes for them to pay me. Or say they drop by with their card.

I may assume they're paying by cheque, then they produce a card. How do i convert an invoice previously written? 

The principle sounds good, but how will this work? This is a HORRIBLE SYSTEM. It should not be this hard.