Sir,
For your ready reference, the provisions of Section 142(2) (a) and (b), which are applicable in your case are reproduced below:-
(a) where, in pursuance of a contract entered into prior to the appointed day, the price of any goods or services or both is revised upwards on or after the appointed day, the registered person who had removed or provided such goods or services or both shall issue to the recipient a supplementary invoice or debit note, containing such particulars as may be prescribed, within thirty days of such price revision and for the purposes of this Act such supplementary invoice or debit note shall be deemed to have been issued in respect of an outward supply made under this Act;
(b) where, in pursuance of a contract entered into prior to the appointed day, the price of any goods or services or both is revised downwards on or after the appointed day, the registered person who had removed or provided such goods or services or both may issue to the recipient a credit note, containing such particulars as may be prescribed, within thirty days of such price revision and for the purposes of this Act such credit note shall be deemed to have been issued in respect of an outward supply made under this Act:
Provided that the registered person shall be allowed to reduce his tax liability on account of issue of the credit note only if the recipient of the credit note has reduced his input tax credit corresponding to such reduction of tax liability.
Moreover, contract clauses may be referred regarding tax, if it is to be inclusive or to be levied separately. If nothing regarding this is mentioned then refer tender and its clauses. If nowhere it is clarified then the valuation shall be as per Rule 35 of CGST Rules, 2017.
With regards to ITC, if your main contractor and not sub-contractor, ITC is not available.
I invite our experts to correct me, if mistaken.