Laurie Santos describes the study in this talk here. Of course, in order to study how monkeys use money, first they have to be taught to use money in the first place. And once monkeys understand the value of money, they engage in thievery (as described in the talk above) and even prostitution.
If you don't want to watch the whole video, the Freakonomics guys describe the same experiment here, including:
Something else happened during that chaotic scene, something that convinced Chen of the monkeys' true grasp of money. Perhaps the most distinguishing characteristic of money, after all, is its fungibility, the fact that it can be used to buy not just food but anything. During the chaos in the monkey cage, Chen saw something out of the corner of his eye that he would later try to play down but in his heart of hearts he knew to be true. What he witnessed was probably the first observed exchange of money for sex in the history of monkeykind. (Further proof that the monkeys truly understood money: the monkey who was paid for sex immediately traded the token in for a grape.)