I will buy 2 779 531 000 000 IOTA (1 000 000 OLD IOTA) for 20 BTC. Escrow only.
1 000 000 OLD IOTA != 2 779 531 000 000 IOTA. That number was
rounded. Just FYI.
Could you please explain?

If 1 "old" IOTA is now equal to 2 779 531 "new" IOTA, so 1000000 * 2779531 = 2779531000000