A professor stole my dissertation topic. Now what?
In two of the answers, people just suggest you to move further and forget about what happened. I disagree with it and think that if you truly believe that people took advantage of you (in my answer I assume that you described the situation in a correct way), you should stood up for yourself and be firm.

I do agree with D.W.'s answer that your chances are not high, and not all wrongs have a good remedy, but I think that you have to report it because: 


how the next potential students of A and B know that they are not people who should be trusted?
when you hear a shot and a sound of breaking glass in the night you call a police, not because you think that your information is enough to find a culprit, but because you do not want this behavior to continue. Whether they will be able to find a person who did it and whether it is really worth of opening a crime case is not your responsibility.


Sorry, but I am unable to give you a suggestion as to how approach this situation, but please do not give up.