KHARTOUM, April 29 (Reuters) - Sudan’s central bank on Thursday relaxed restrictions to allow commercial banks more flexibility in selling dollars to clients, after the country’s pound currency weakened against the dollar on the black market for the first time in months. Sudan devalued the pound in February and put in place a managed currency float mechanism to allow commercial banks to take over the currency market and stamp out informal trading that had been driving the currency to historic lows. The largely successful 85% devaluation was challenged only last week when dollars began to appear on the black market once more at only 10 pounds more than the official price of 388. Bankers said commercial banks were not selling sufficient dollars to importers, creating an imbalance and forcing them to tap informal traders for the needed funds. Officials have said that the import of non-essential goods would be restricted, but traders say such limits have not been applied. In circulars to banks and exchange houses, the central bank said it was extending the window within which banks can sell dollars, and would provide foreign currency notes to banks if needed, provided the amounts were repaid with its correspondents abroad. Exchange houses could repay in local currency, it said. Under rules issued at the time of the devaluation, customers can only buy foreign currency for central bank approved purposes such as travel, education, medical treatment or the import of goods, and only with supporting documents. (Reporting by Khalid Abdelaziz; Additional reporting by Nafisa Eltahir; Writing by Nadine Awadalla; Editing by Richard Chang)