Business is business and friends are friends.
Making someone a deal now only to see them sell it later or sell babies later for high prices without giving back, not necessarily to you but not passing on the good you gave them with the discount, can stress a friendship.
Like Judy said, it's really a case by case thing without any hard and fast rules.
Expect nothing but heartache and you won't be disappointed. You may not be disappointed, and if you're not, you're one of the lucky ones. It's VERY difficult to do business with friends.