While I do not have access to your contract and your questions does not state, I am assuming that this is a FFP Construction contract. With this being the case, FAR 16.202-1, a firm-fixed-price contract provides for a price that is not subject to any adjustment on the basis of the contractor’s cost experience in performing the contract. This contract type places upon the contractor maximum risk and full responsibility for all costs and resulting profit or loss. The contractor is required to deliver exactly what the contracts states, nothing less and nothing more.
As far as the excess supplies, first this is a relationship between a prime and a sub which we have no privity of contact. Assuming that you did not modify the contract to include/require delivery these excess supplies and the contractor completed the contract as written, the government has no right to anything beyond the requirements in the contract including excess supplies.
If the contractor was late completing the job, then the contactor could be held liable for liquidated damages (LD’s) unless an excusable delay existed under FAR 52.249-10 for the late delivery of the supplies. If you charged the contractor with LD’s then you could trade the excess supplies as potential consideration for the LD’s (However, I do not know the dollar value of the LD’s nor the excess supplies). I say this in theory.