Your contractor (possibly) is the one losing money.

The contractor has a right to make money on something he buys and resells. It's kind of ludicrous to sit on that money and risk being stiffed by the customer and NOT take that profit. (Maybe you paid him directly upfront for the materials in this situation, so he doesn't care. Maybe he got the slab for $1500 and is still making money.)

The bottom line is there's traditionally a chain of markup. The manufacturer obviously makes a profit over cost of goods sold, they usually only sell in bulk to a supplier/wholesaler, who usually only sells to a retailer. Some companies protect the supply chain, some don't care and manufacturers sell directly to retailers, suppliers sell directly to customers. And some suppliers take on both roles, but still protect the supply chain by selling at retail rates to customers and wholesale rates to contractors/retailers.