For a while, the Russian ruble was pegged, by the SU, to the US dollar. 1 ruble = 1 dollar. But the price on the black market was....different of course, because the market always has its own logics. Anyway, it's extremely hard for the average Soviet citizen to go obtain a passport, let alone go abroad, so only a frowned upon, pariah "class" of "outlaws" would need foreign currency for buying and selling things. Those "outlaws" included "illegal" merchants(everything not distributed by the State was illegal), criminals, smugglers, youths who would like some jeans or music discs.... Sometimes one was all of the above.
As going abroad was strictly forbidden, only "officials" and their families could and when they did, they could exchange a lot of those worthless ruble for the USD, at Russian embassies in foreign countries. Daughter of President Brezhnev did involve in such large scale currency smuggling activities. Then there was a State Bank. Everything else was people's shit, but the Bank was the property of the State. From wikipedia: "The Soviet state used Gosbank, primarily, as a tool to impose centralized control upon industry in general, using bank balances and transaction histories to monitor the activity of individual concerns and their compliance with Five-Year Plans and directives. Gosbank did not act as a commercial bank in regard to the profit motive. It acted, theoretically, as an instrument of government policy. Instead of independently and impartially assessing the creditworthiness of the borrower, Gosbank would provide loan funds to favored individuals, groups and industries as directed by the central government. As part of Mikhail Gorbachev's perestroika program, other banks were formed, including; "Promstroybank" (USSR Bank of Industrial Construction), "Zhilstoybank" (USSR Bank of Residential Construction), "Agrobank" (USSR Agricultural Bank), "Vneshekonombank" (USSR Foreign Trade Bank), and "Sberbank" (USSR Savings Bank). "Sberbank" continues to this day as one of Russia's largest banks, retaining senior ex-Gosbank personnel and most of the present Russian government's banking business."

So if a Soviet wanted to lend money, he obviously would go to the black market, and ask some criminals(often well- connected to the government) to lend him money. One of the ultimate goals of Communism, at interpreted by the State, was to eliminate money. Everything needed to be rationed. No wonder the Soviet Union had more than 20 years of stagnation. Toward the end of the Soviet Union, crime and smuggling flourished. Western sanctions coupled with mismanagement had forced Gorbachev to "reform" to obtain capital, which eventually lead to the collapse of the SU. Read more "Oil and grain: the collapse of the Soviet Union" by Yegor Gaidar http://www.aei.org/wp-content/uploads/2011/10/20070419_Gaidar.pdf