Say's Law - production IS demand - applies only in a fixed supply monetary system.

In a fiat system (which, by definition is inflationary AND deflationary), new money (debt) creates (counterfeit) demand (see quantity theory of money). This is inflation (of the money supply).  When the debt is extinguished, demand is reduced, i.e., the money supply undergoes deflation.

In a fixed money supply system, only new production can stimulate increased demand, as demand created artificially by the creation of more money - inflation of the money supply - is impossible.

NB:  Rising prices are not in and of themselves inflation, and falling prices are not in and of themselves deflation.  It is a shame that this is not more widely understood, even by many "economists".