First of all please note that you should be working with a BOQ wherein the total work to be carried out is briefly mentioned, then for details you can refer to specifications wherein the total work is mentioned in detail.
The contractor cannot dictate his terms but has to work as per the parameters laid down by the client or clients consultants.
Now if the contractor is trying to play mischief then bring it to notice of your senior and show him the malpractice followed by the contractor, if senior doesn't co-operate then you need to bring it to the notice of his senior or anyone in your organization who is a responsible person, but do it tactfully.