What Changes Value in Volume One?

In volume one of his book, he assumes that commodities are sold at their values and that the circumstances in which this takes l do not change. He is actually going to write a whole book where technological change is assumed to be constant. We shall also ignore any changes of value that may occur in the course of the cyclical process. So why on earth write a text which ignores the foundational findings of volume one? i think the answer lies in the idea that there were assumptions about how this flow occurred, which are hidden, and he wants to extract them.