(b)if a person whose interest in expectancy in the property so purchased, acquired or extinguished becomes entitled to an interest in part only of that property, this provision shall apply only to such part of the debt or incumbrance as bears the same proportion to the whole debt or incumbrance as bears the same proportion to the whole debt or incumbrance as the value of the part of the property to an interest in which he becomes entitled bears to the value of the whole of that property.
