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The bank offered $40 million and said it had sold $38.8 million at a cut-off price of 7.1401 pounds a dollar, unchanged from its last sale on Thursday.
The rates at which banks are allowed to trade dollars are determined by the results of central bank sales, giving the bank effective control over official exchange rates.
In the unofficial market, the pound was trading at 7.68 to the dollar, one trader said, weaker than Thursday's 7.65 to the dollar.