(2) An assignment of a purchase agreement, or of an interest in the purchase agreement or of an interest in the property described in the purchase agreement, is not valid unless it is in writing, subscribed by the holder of the purchase agreement and filed with the county clerk of the county in which the land is situated. [Amended by 1983 c.310 §15; 1991 c.67 §65; 2005 c.243 §5]