FRANKFURT (Reuters) -Uniper, Germany’s main importer of Russian gas, on Monday said it would be possible to pay for future supplies without breaching European Union sanctions even though Moscow has demanded payments in roubles. Russia’s demand for gas payments in roubles has been a major headache for European customers, mainly due to the opaque nature of the request and its potential to violate existing EU sanctions on Russia. “According to an initial and therefore still non-binding assessment, we still consider a compliant future payment processing to be feasible,” Uniper said in an e-mailed …