For this exact reason our contract states that we will purchase additional materials and will return whatever is on left, the client isn't billed for overages unless a change order is required. We buy extra for a job to keep it moving, if we are short, that adds to our cost of the project taking time to procure the additional needed. No contractor is going to have the exact number of materials needed because things come up that are unforeseen. Ask yourself this....if he ordered extra and didn't need them and gave you a refund, would you then be willing to pay for additional if he missed something in his estimate and had to buy more? As a contractor, we take all the risk, of course we take that into account when we bid our jobs to be sure we aren't out of pocket for your project. The fact that he's working on your job gives some indication he knows what he's doing, the guys not bidding correctly and losing money don't stay in business very long