The United States government on Monday gave American customers of Russia’s biggest aluminum producer more time to comply with sanctions. This move set aluminum prices into a tailspin but lifted shares of the company, United Company Rusal Plc. Aluminum prices on the London Metal Exchange tumbled more than 8 percent after the U.S. Treasury Department announcement, which gives Rusal longer to sell off large quantities of aluminum it had been stockpiling in the wake of sanctions.