The bounty hunters that Huck meets give him money because the feel sorry for him. They feel sorry for him because they have "bought" his story implying that his dad and sister are on the raft, suffering from smallpox. Huck has made this story up to prevent them from going to investigate the raft.
I suppose the irony here is that they are acting in a very humane way when they are in a fundamentally inhumane "profession." They are bent on finding runaway slaves and ruining their lives, yet they are humane enough to want to help white people they don't even know.
The bounty hunters give Huck money because they believe it to be a moral act: a family they can relate to (a presumably white father and daughter) is suffering from a dreaded, potentially fatal, and highly contagious disease, and they feel obligated to somehow help the son who is looking after them. There are at least two ironies in their reaction to Huck's story. The first is that they find Huck's story credible, when he is, in fact, lying about who is concealed on the raft. Another irony is that the bounty hunters do not recognize that in running down escaped slaves they are engaging in an act that is legal, but not moral. The fact that they float the money over to Huck on a board because they do not want to get close enough to imperil themselves suggests another target of Twain's multi-layered satire: our tendency to throw money at problems while keeping ourselves at a safe distance to assuage the guilt we may feel about others' misfortunes.