You're probably thinking about stuff like bug bounty programs. The reality though is that even if you had found a
security critical bug AND the company had a
bug bounty program in place, they may just not pay. Most companies are very stingy and don't like to pay even for good bugs. They're also not forced to pay either, right. Few bug hounters actually live on it since the rewards aren't as high as most people seem to think and because of the uncertainty of getting paid or not.
Now in your case, you found a miscalculation bug of the taxes or something like that, but nothing security critical. So there's no security hole, no exploit, no bug bounty program (I suppose? Not many app companies have them), I don't think a bounty is in sight, sorry