The Commissioners shall refund to the Government of Northern Ireland the amount of the VAT charged on the supply of goods or services to that Government F1... or on the importation of any goods by that Government F2..., after deducting therefrom so much of that amount as may be agreed between them and the Department of Finance and Personnel for Northern Ireland as attributable to supplies F3... and importations for the purpose of a business carried on by the Government of Northern Ireland.