あ means almost nothing here, but is added for mild emphasis. It's the same あ after な at the end of this sentence. It has nothing to do with ああ ("that way").
いうことない is 言うことはない ("I have nothing to say", "There is nothing to complain").
そのうえ賞金もらえりゃあいうことないなあ。
= そのうえ、賞金もらえれば、言うことはないな。
If I can even get a bounty, there is nothing to complain, huh?