Reading over everyones responses. Yes i see the issue of this being highly exploitable. Yes i've noticed that. This is only an idea and will be modified. So that it can prevent being exploited.
I played a vampire game that had a bounty system in it. It worked pretty well, but overall that game sucked.
Pay to have someone pinned once? Seems pretty pointless to me. Maybe if the bounty was an ongoing thing where every time they are pinned a percentage of total bounty is paid. Perhaps the percentage to vary based on number of hits it took for each pin.
What if you were to do it like: The player who sets the bounty also sets the reward, it comes straight out of their own money. Then if someone successful pins all their troops they will receive 66% of the bounty and the rest goes into hyper space to prevent people taking advantage of the system.