There are no such thing as "contra-charges" under the contract. If the works are de-scoped then these should be instructed and notified as a compensation event. The Subcontractor then should have the chance to price how much they think it would have been and the Contractor can consider if they agree with the quotation and assess themselves (in accordance with the rules) if they don't agree.
The bad news is that it would be assessed at the cost of what it would have cost them at the point it was de-instructed - which might have been more than it would have been tendered for (but if it wasn't removed it would have cost them more to install anyway).
The bad news is that it would be assessed at the cost of what it would have cost them at the point it was de-instructed - which might have been more than it would have been tendered for (but if it wasn't removed it would have cost them more to install anyway).