The Federal Court has dismissed an application by Macquarie Group to have a tax ruling on an offshore banking unit overturned. The ruling gives the Australian Taxation Office the right to issue an amended assessment relating to the treatment of expenses in the OBU.In a ruling handed down on Tuesday, the Court said Macquarie had "no reasonable prospect of success" in the case, which was concerned with a retrospective application of the ATO's view on the law on the allocation of OBU expenses.OBUs are subject to a concessional tax rate, which is an effective 10 per cent income tax on eligible activities and an exemption from interest withholding tax. The purpose of the OBU regime is to encourage offshore financial transactions between non-residens to be conducted through Australian institutions.In the May Budget the Government announced a review of the OBU regime, aimed at closing what it described as loopholes in the system.Macquarie claimed in its application that the ATO had changed its view on the law relating to OBU expense allocations.The court said it was not possible for Macquarie to challenge the ATO decision because it was not a "decision under the enactment and does not directly affect legal rights or obligations."