The Changes Clause, 52.243-1 ALT II is in the contract. The ALT II clause, specifically states
"Alternate II (Apr 1984). If the requirement is for services (other than architect-engineer services, transportation, or research and development) and supplies are to be furnished, substitute the following paragraph (a) for paragraph (a) of the basic clause:
(a) The Contracting Officer may at any time, by written order, and without notice to the sureties, if any, make changes within the general scope of this contract in any one or more of the following:
(1) Description of services to be performed.
(2) Time of performance (i.e., hours of the day, days of the week, etc.).
(3) Place of performance of the services.
(4) Drawings, designs, or specifications when the supplies to be furnished are to be specially manufactured for the Government, in accordance with the drawings, designs, or specifications.
(5) Method of shipment or packing of supplies.
(6) Place of delivery.
This clause, 52.243-1 ALT II, is used for specific changes and a mistake in the price is not one of those changes. “
A bilateral modification should be used per FAR 43.103 (a)Bilateral modification (3) Reflect other agreements of the parties modifying the terms of contracts.
On the SF 30, modification form, block 13 requires you check A, B, C, or D. A is used for a Change Order per the Changes Clause which does not work here. B is used for administrative change or a unilateral modification which will not work here. You can use C or D because you will be using a supplemental agreement or a bilateral modification since there needs to be an agreement on money. I would suggest the use of D(Other) and state: bilateral modification per contractor's quote dated 28 Sept 2014. In block 14, description of the modification, language will indicate the change in the two CLINs that should be decreased like CLIN 0001 decreased from $100 to $90 and CLIN 0002 decreased from $250 to $200. Change in the total price of the contract from $500 to $460. The contractor will sign the bilateral modification showing agreement to the decrease in contract price.