Wait what? Did you read the manga? This isn't possible.
And his bounty won't be higher than Blackbeard's.
Edit: Well shit, never mind, looks like it's been answered.
I don't think it's actually that much, but It's not entirely impossible for it to be in the 2 billion range, because Blackbeard had 0 bounty at the beginning of his rise to infamy, and after taking down the Whitebeard remnants it spiked to slightly over 2 billion.
If you look at all of Luffy's increases. it's never once been equivalent to that of the opponent he's defeated.
He got 30 million for defeating Buggy, Don Kreig, and Arlong, who were combined worth 52,000,000
Then he got a 70 million increase for defeating Crocodile, who was worth 89,000,000
Then he got a 200,000,000 increase for declaring war against the WG, and defeating Lucci, but Lucci didn't have a bounty.
Then he got another 100,000,000 for defeating Moriah and causing havoc throughout Impel Down and Marineford. Moriah's frozen bounty was 320,000,000.
Then he got a Billion bounty increase for defeating Cracker and Katakuri and wrecking Big Mom's territory. Their combined bounty was 1,917,000,000
So Marco and the WB remnants having relatively higher bounties than the other Yonko Commanders makes sense considering how much Blackbeard's bounty became after defeating them.