When addressing an issue like this the best tactic is to avoid making accusations that you are not prepared to take to the bitter end.
Right now you do not know that this person is stealing, you suspect that. So rather than making an accusation with no evidence to back it up and forcing a confrontation which you are not properly prepared for, you should investigate. Do not seek to understand why the bonus went to that manager, seek to understand why it did not go to you. The result should be you understand both in the end anyway, but if you go looking to understand why the other manager got the bonus you are more likely to set the other managers on the defensive.
My first stop would have been ask your manager why the bonus went to this manager instead of you. If the answer does not make sense to you ask them to clarify. Ask if the policies are written down somewhere. If they are ask them to take you through it. If at any time the manager asks if you are trying to make trouble or cause problems you must insist and should always approach this as just trying to understand the policies and how the bonuses work.
If the manager does not have the answer(s) and does not seem interested in helping you find that answer then ask them who you should talk to about it to understand. If they do not point you to someone try to climb up the chain yourself. If this person is embezzling then your employer could end up being held liable, so someone is probably going to take an interest at some point, that or they will be able to explain how it works to you.
If you get to the point where you understand the policies or come to the end and still do not have good answers and no one is looking into it then you have a choice to make. You can either suck it up and allow this to continue, or you can try to take action. If you take action outside of your employer it will have repercussions for you at work. It could cause you to be fired, or just treated as an outcast.
Your options for taking actions are to go to the police or district attorney. There is a good chance unless you can feed them a winning case they are just going to tell you this is a civil case and not do anything or just do a cursory investigation before dropping it. You can take it to small claims court yourself or you can get an attorney to help. Chances are 100 is not worth it but if this continues and that number is 10000 in a year it might be worth it. And at that point your attorney could do the investigation and it may lead to criminal charges. But right now there really is little to no reason for you to go forward with taking this outside your company unless you can get several other people to say they have done this to them and you can go together as a group.