Porter entered into a contract with Hip Sports Inc under which Hip agreed to sell Porter's basketball shoes as its agent. Hip was authorized to enter into contracts to sell the shoes on Porter's behalf and Porter agreed to pay (i) a commission of 10 percent of the sale price of the shoes, and (ii) all of Hip's advertising expenses. 

Hip represented several other lines of products. Hip planned to advertise Porter's shoes through a flyer campaign. The going rate for having flyers produced was $5000. However, Hip was able to negotiate a discount of $1000 because it did a high volume of business with the flyer producer for its other clients. Hip saw no reason to pass that benefit on to Porter since it was granted on the basis of all the business that Hip did with the flyer company-for Porter and for other clients. Hip therefore charged Porter the full $5000. Is Hip liable to give Porter the benefit of the discount?