Resale of fungible goods from commingled mass

If a purchaser gives a certificate with respect to the purchase of fungible goods and thereafter commingles these goods with other fungible goods not so purchased but of such similarity that the identity of the constituent goods in the commingled mass cannot be determined, sales from the mass of commingled goods are deemed to be sales of the purchased goods until a quantity of commingled goods equal to the quantity of commingled purchased goods has been sold.