Regarding merchant quest rewards i found out how to change them, code responsible for it is inside Common_store.c file.
iMoney equals quest reward.
Code:
[...]
iQuantityGoods = iQuantityGoods - rand(makeint(iQuantityGoods/3)) - 10;
iMoney = makeint((iQuantityGoods * sti(Goods[iTradeGoods].Weight) / sti(Goods[iTradeGoods].Units)) * (4+rand(3) + GetSummonSkillFromNameToOld(pchar, SKILL_COMMERCE)) + 0.5);
pchar.CargoQuest.iTradeGoods = iTradeGoods;
pchar.CargoQuest.iQuantityGoods = iQuantityGoods;
pchar.CargoQuest.iMoney = iMoney;
pchar.CargoQuest.iTradeNation = iTradeNation;
pchar.CargoQuest.iDaysExpired = 25 + rand(5);
iMoney equals quest reward.