On the surface, the answer here is simple: "Read your contract." One of the key elements of a reasonable contract between a hunting party and an outfitter should be a section that covers payment schedule and cancellation policies. You must know all of what you are agreeing to before you sign the contract.
Agreements are all over the board on cancellations and postponements. Some outfitters have a no cancellation policy -- if you don't make the trip you lose your money. Some invoke a cancellation fee so that you forfeit a certain percentage of your payment for a hunt that you cancel. At first these might not seem reasonable, but they are essential to any outfitter. He/she only has a limited number of tags, area and/or hunting time. When you commit to making a trip, those resources are completely lost to the outfitter if you bail on the hunt and there's usually no way to recover, especially on short notice.
If you cancel your hunt with enough time for the outfitter to rebook the trip via a "fire sale" or some other method, then usually, they will refund part of the money or allow you to use a portion of the money you've paid to make a hunt in a future.
Cancelling or postponing a hunt is another situation in which you must remember that your outfitter is a businessman or woman. They are making their living off of selling these adventures. They understand that "things come up" as they do in their own lives, but they can't afford to fund those unexpected situations in your life.
Again, if you need to cancel or postpone:
STEP ONE -- read your contract.
STEP TWO -- talk it through with your outfitter, but don't go in expecting him or her to break the bank for you.
STEP THREE -- be grateful if he or she is willing to negotiate.