The antonym of “greedy” is “generous”. Is there a word for “not greedy” (one who is content with what he has)?
(one who is content with what he has)
But you just used the perfect word to describe a person who is content with what they have: Content. (Hence also, satisfied, sated).
Though to go to the far end of the spectrum so as to be an opposite rather than just different, you'd want ascetic to cover someone who consciously abstains (abstemious, abstinent) from the pleasures the greedy person seeks out.
Satisfied means content. Its etymology traces back to a word for "enough," so someone who is satisfied has enough and does not yearn for more.
I would use altruistic.
Greedy is almost directly a synonym for selfish (using Google's definition):
having or showing an intense and selfish desire for something...
Altruistic is almost directly a synonym for selfless (emph. mine):
showing a disinterested and selfless concern for the well-being of others; unselfish
The word for "not greedy" is "ungreedy"!
There are various near-synonyms for "content", which is a slightly different thing as others have pointed out. Someone can be both greedy (as a character trait) and content with what they have right at this moment (as their current state). Perhaps they're sitting in a heap of cake.
If nirvana were not so poorly understood in English, then it would be almost perfect.
A significant, and necessary, aspect of nirvana is freedom from desire, which is difficult to distinguish from your parenthetical, "one who is content with what he has."
Nirvana means "to blow out", as in "to blow out a candle's flame." In Buddhism, one of the three flames a Buddhist seeks to blow out is the flame of raga, which is translated as attachment, passion, or desire.
For more insight into nirvana, consider that in Hinduism and other Indian philosophies, the attainment of (or possibly, state of) nirvana is moksha, which is a sophisticated idea but notably includes the requirement of artha. Artha is the attainment of, or process of attaining, all things that are necessary to live. Because artha is one of four goals, it must be balanced against the other three, and if one were to live in excess of artha, then one would violate one or more of the other goals. In total, the concepts of artha and moksha support the idea that nirvana must include a condition of "not greedy."
Nevertheless, if your audience is from a culture born of the Hellenistic-Roman world, then my guess is that they will misinterpret nirvana to mean something like salvation or heaven, so your true meaning may be lost--despite the noble efforts of Hermann Hesse.
Given the broad nature of the question as it has been asked, I offer the word fair (from http://dictionary.reference.com/browse/fair, "free from bias, dishonesty, or injustice") as in this example:
Being greedy, he always took the most lucrative deals for himself.
Being fair, he shared the most lucrative sales around the team.
minimalist or minimalism--a broad concept applied to many human activities. Minimalism emphasizes simplicity, The term is applied to styles in art, music, architecture, automobile design (e.g. Colin Chapman). It can be applied to government policies which de-emphasize elaborate regulations. On the personal level, minimalism applies to a life-style that minimizes clutter (not simply clutter in the home but clutter in one's personal affairs--romantic, financial (minimalist trading techniques on Forex), spiritual, material, travel and more. A minimalist life-style is much like my own.This minimalist has a decent income, but owns no house, no phone, no car (rides a bicycle), has no insurance (although 75 years old), does not visit doctors or dentists because he doesn't need them. He employs his own cures for ailments aided by an immune system kept strong by rigorous exercise, simple but healthy diet and intellectual curiosity (foreign language study). He has no wife and no children. He travels but doesn't need much money since he stays abroad for long periods in foreign countries working at jobs abroad. He thinks money is not necessary for world travel--an adventurous spirit is necessary.This is extreme, but you see the general form of minimalism.