In the below scenario, is Todd entitled to the two amounts he's claiming for James? What common law of contracts can I incorporate?

James hires a sub-contractor, Todd to install the underfloor heating pipes of a house for a fixed price of $45,000, payable on completion of the work.

Todd starts work as agreed, but part way through it, realises he has miscalculated the floor area and underquoted for the job. He needs additional materials which he can't afford.

James is worried about the project being held up, so he says he will pay Jones $16,000 extra if Jones continues working on the job. He gives him $10,000 immediately, and Jones orders the materials.

Two weeks later, James has still not paid the outstanding $6,000, so Todd stops work and claims both the original $45,000 and the additional $6,000.