You would want to sum up the loads by contract with SUMIFS(). This will sum up a range with a condition of the load matching the contract. Then you can easily subtract the load total from the contract.

Since you have the dropdown for contract, that should go in column A (doesn't matter, I'm just using that for illustration). The load amount goes in column B.

=SUMIFS(loadrange,contractrange,contract)

or

=SUMIFS(b4:b100,a4:a100,a1) for example. Where a1 contains the contract name you want to sum the loads for.