Attacker, gets some experience when he attack someone and can also get some money after he won.. and loss some experience when he loss the fight. The defender should also get something in return when he won the fight e.g. some stamina or experience

I think its perfect how it is, Geting Experience would be bad for those level sitters., becouse if someone wanted them to level they just had to keep attacking and loseing.

btw, when a battle is over, both winner and loser recover some STM, the winner recovers more than loser so the defender already gets STM if they win