Is the engineer entitled to demand payment from the client for the excess square footage of ceramic tiles that the client was unable to return to the store due to the engineer's delay in installation?

This issue is closer to arbitration and requires details and hearing from both parties. If the questioner is an agent for the engineer in purchasing ceramics, then the purchase belongs to the principal, not the agent, and ownership and contractual rights pertain to the principal. If the principal instructs his agent to purchase a specific quantity, then whatever remains of this quantity with the principal is his property and responsibility, and he cannot compel the agent to take it and pay its price.