Project Manager instructs a change to the Scope to include additional work before. Now, Client had decided not to undertake the additional items of work at the current time, what provisions are contained in the contract to assess the programme impact prior to being committed?
If I am understanding your question correctly, you have 2 separate compensation events, the first for the introduction of the new work, the second for the deletion of the new work.
How to assess will depend upon the timing between the 2 instructions and what measures you commenced for the first (including any programme adjustments) before the second was instructed. The two should be assessed in the standard manner but if they were instructed relatively close together they may neutralise each other. There is an argument that any change in the Completion date due to the first cannot be reduce by the second but I think this needs to be consider pragmatically, again, based on the timing of the instructions.