Fix NPC trade and item rewards

[Coolthulhu]

Bug:
Asking NPC for an item reward for a mission can bug their owed value.
Exchange (trade with friends) requires it to be 0, otherwise it requires either explicit confirmation or refuses the trade.

Fixed both cause and symptom:

• Trade with friends ignores owes value
• It's no longer possible to ask for item rewards with friends. They give stuff for free, so it simply doesn't make sense for them to give it as a reward.