Under the barter system of exchange, acts of sale and purchase of an individual were to occur at the same point of time. To buy a thing, an individual must at the same time sell something needed by the other person. Also, sale and purchase by an individual must be of equal value.
But with the introduction of money (as a medium of exchange.), an individual can buy a commodity or service with money without selling anything at the same time. Likewise, he can sell a commodity for money without buying anything at the same time, Thus, with the introduction of money, acts of sale and purchase are separated.