When two people each have items the other wants, both people can determine the values of the items and provide the amount that results in an optimal allocation of resources. Therefore, if an individual has 20 pounds of rice that he values at $10, he can exchange it with another individual who needs rice and who has something that the individual wants that's valued at $10. A person can also exchange an item for something that the individual does not need because there is a ready market to dispose of that item.
As Orlove noted, barter may occur in commercial economies, usually during periods of monetary crisis. During such a crisis, currency may be in short supply, or highly devalued through hyperinflation. In such cases, money ceases to be the universal medium of exchange or standard of value. Money may be in such short supply that it becomes an item of barter itself rather than the means of exchange. Barter may also occur when people cannot afford to keep money (as when hyperinflation quickly devalues it).
Whether or not one agrees with such broad claims, it’s worth noting that monetary debt, a byproduct of currency, has regularly been used to by some groups to manipulate others. Thomas Jefferson, for instance, suggested that the government encourage Native Americans to purchase goods on credit so they’d fall into debt and be forced to sell their lands. Today, black neighborhoods are disproportionately plagued by debt-collection lawsuits. Even after taking income into account, debt collection suits are twice as common in black neighborhoods as in white ones. $34 million was seized from residents of St. Louis’ mostly black neighborhoods in suits filed between 2008 and 2012, much of which was seized from debtors’ paychecks. In Jennings, a St. Louis suburb, there was one suit for every four residents during those years.