The best way might be to use a trigger to turn on bounty for each player, using the "Player Flag" action, during Map init. Then change each unit to give whatever amount of bounty you want it to give.
Alternatively, you could make an easy trigger that detects when a unit dies and gives +2 gold to the owner of the killing unit, unless that owner is also the owner of the dying unit. It'd be small and simple, but it wouldn't show the bounty art above the dying unit's head.