You may want to investigate how long your chosen payment processor (PayPal/Google Checkout/etc.) can hold a pending payment to await manual approval before it would otherwise expire.
If they can authorize a payment amount without auto-completing the actual payment immediately, and keep it in pending status long enough for you to finalize and launch the product (and upload a file to be downloaded for the product in Seller Admin at that time) then when you manually approve the early payments that were left on hold in pending status, our system would be notified of the completed payments and process the sale at that time, including issuing each buyer their download link.
Otherwise, you can take orders and allow payment to complete automatically as usual, then use our Updates feature after you upload the final file for your product to issue all buyers of the product a fresh download link:
(However, buyers who choose to opt out of joining your mailing list will NOT receive any Update emails, so you need to be very emphatic that they must join your list during checkout!)
Finally, you can always use Seller Admin > "Send free download link" to send links to individual buyers.