Save the game before you buy the item that you want, then buy it. Make sure you have enough Gald for it. After you buy it, go to the inn. Talk to the innkeeper, and select "Trade" from the list of options. Make sure your friend does the same. Then, trade your friend the thing you just bought for a worthless item. After the trade, reload the game from the previous save point before the purchase. Your money will be the same as before you bought the item. Go back to the inn, and trade your friend a worthless item for the item that was just traded. You will now have the item but will not have spent any Gald.