if there were previous sales of this mutual fund for which you used the average cost basis, that is the method that must be used for all future sales. . when you set up the account you may have indicated on the paperwork for the broker to use average cost for sales.
none the less, if there have been no previous sales of this fund using the average cost method you can use the FIFO method. However, then FIFO must be used for all future sales of these shares with an exception, (if the average cost has never been used) then instead of FIFO you can tell the broker to sell xxx shares purchase on XX/xx/xx (specific identification of shares you still hold)
