Reward: 1000 or 2500 Gold.
Behind one of the Oakvale houses you'll meet a guard who wants to hire you to kill a dangerous assassin in Twinblade's Tent. Go there. The bandit will ignore your presence unless you take out your sword. Then, he will double the stake and try to hire you to kill the guard, who turns out to be his brother. If you refuse, he will add another 500 Gold to tempt you. Now the decision is up to you - kill either the assassin or the guard in Oakvale.