In 1997, a seller and buyer enter into an agreement for the purchase of industrial ball bearings. Buyer agrees to a minimum purchase of ball bearings each year. The buyer never purchases the minimum amount determined by the contract. The plaintiffseller does not demand, as it could under the contract, the difference between the actual amount of ball bearings purchased and the minimum amount required under the contract. As such, the parties negotiate a new contract in 2000, lowering the minimum amount of ball bearings to be purchased. From 2000 to 2004, the buyer never purchases the minimum amount; the difference is never paid. Finally, in the sixth year of this new contract, the plaintiffseller sues for the fifth and sixth year underpurchases. Defendant-buyer argues that plaintiff-seller has waived its right to collect the underage at this point in time. Have they?