Changing ok f prices while you are on lay-by

Hi my name is Reginald Majola I want to make a complain with the hardware fair price in Kranskop KZN .my wife made a lay-by for building materials in October 2020 the lay-by duration is one year if my wife come to pay the last amount yesterday the manager of the hardware Price's are increas so the still owning the more money by that time spend her(R55) per trip she go back without material so what I need to know is that is there such a thing that price increase while you already has an agreement is what I need to know