The funk brothers were actually never called bounty hunters in the manga, they are underworld assassins tho.
They may not be mutually exclusive, but they’re also not one and the same
True, but they were described for hunting down big bounties, and Daz Bonez was also called an assassin at some points as well.
Personally, I could see Daz getting his bounty after the break out. Or it could just another example of the world government trying to supress what happened in Alabasta. Luffy beat Crocodile to get a 100 million berri bounty, which in the first half of the grand line is a really big deal(there were people in Jaya who claimed they'd never met anyone with a bounty as high as Luffy's). However, 100 million berri bounties only belong to level 5 in impel down, and Crocodile was a level 6 inmate.
Either way, I'm not sure whether it pedantically valuable. I think that Mr 2's bounty is probably a bit understated when he was arrested, and adjusted for when they imprisoned him.
Even if Daz did have a bounty of 75 million during Alabasta, you could still argue that it was a matter them downplaying the events of Alabasta.