This is an interesting book that I think exceeds his more recent and more honored titles such as 'Camouflage.'
Buying Time is about a future where life can be extended via a special medical procedure, but the technology is controlled by an organization that is outside the control of a government or other regulatory body. The organization ensures that population does not get out of control by limiting the use of the life extensions. They also have safeguards against very old people from getting too powerful over time.
Like many of Haldeman's other books, there's some gunplay in here and some great characters. If I were composing a list of classic Joe Haldeman books, Buying Time would be in the top three.