How do you buy Castle Fairfax in Fable 2?

Once you helped Farmer Giles with Reaper, and done the the Blind Date quest ( you help to find a date for his son which you find out he is gay and he and his dad leave for the city after you try to find him someone) you can buy the Giles Farm by going up to the building sign and pressing A. You will have to save some money for it. Once you buy it, you will not be able to sell it. But, you have to buy the farm in order to get the cellar key upstairs and go into the cellar to find the legendary blunderbuss.