The performer shows five opaque envelopes in which are pieces of paper cut to the size of a $100 bill. The performer replaces one of these sheets with an actual $100 bill and seals the envelope. The envelopes are shuffled and the spectator chooses one. The remaining envelopes are burned, and it is revealed that the spectator was lucky enough to have chosen the correct envelope, saving the cash from a horrible (and financially disastrous) fate.
Before the trick, five envelopes are prepared. One has a piece of paper in it, and the other four have $100 bills. The magician says that all of the envelopes have paper in them, but only opens one. He then takes that piece of paper and replaces it with currency, so that when the audience member chooses, it is a choice among envelopes that all contain $100. The trick works automatically from that point.
A less expensive alternative exists for use by performers who do not have sufficient $100 bills available. In this case, put $1 bills in four envelopes and a piece of paper in the fifth and proceed as above. But when it's time for the spectator to choose an envelope, the magician secretly weighs the envelopes by moving them from hand to hand until the $100 envelope (which will, obviously, be heavier) is found. This envelope is then "forced" on the spectator using equivocation ("Are you sure you don't want to change your mind? Really sure? Like to this envelope over here? The extra nice one? Are you sure?" etc.)
Note that this second method works particularly well in the U.K. where £1 is a coin and £100 is a bill.