Imagine you have ecommerce website with product listings, checkout, thank you page etc. Usually "purchase" data is being sent on the thank you page (after payment is done and order data is finalized).
But what if some payment options will not redicrect customer to you website's thank you page? In this case some transactions will be missed in GA because customer journey ends on 3rd party site (for example on the bank page). In my case it's about 30% of transactions.
My question is: Should I sent purchase data previous to thank you page in this case? For example on "purchase" button click in the checkout section.
Theoretically, the bank's payment gateway should have in its configuration the possibility to indicate a page of your website to which to redirect.
If this is not the case, you can check if there is a server-to-server solution integrable system (such as the PayPal IPN to be clear).
Regarding your question, the answer is that surely you can do as you say (it's the same approach used for wire transfers), but you will never be sure that those transactions have actually been paid.