The Contractor is correct, de-scoping works will reduce Defined Cost but not affect the Completion Date, it's up to the Contractor to choose how he uses the time, he can choose to finish earlier and move planned Completion back or re-programme and use the float to finish on or before the Completion Date. Clause 63.3 makes this so as it only deals with delay and planned Completion finishing later than planned Completion shown on the Accepted Programme as a result of the compensation event.
The Contractor has done nothing wrong by using float in the original programme in the way you describe - it is his to use as he pleases. Fairness has nothing to do with it, it's what the contract says. If the Contractor agreed to go along with what you want and the PM introduced additional scope and the Contractor was then delayed to make him late would you feel this was unfair as well?
To my mind more effort should have been put in at the pre-contract stage to ensure the scope was clear and the programme understood so that a realistic Completion Date was set from the outset.
The Contractor has done nothing wrong by using float in the original programme in the way you describe - it is his to use as he pleases. Fairness has nothing to do with it, it's what the contract says. If the Contractor agreed to go along with what you want and the PM introduced additional scope and the Contractor was then delayed to make him late would you feel this was unfair as well?
To my mind more effort should have been put in at the pre-contract stage to ensure the scope was clear and the programme understood so that a realistic Completion Date was set from the outset.