This question needs to be answered using as many legal terms that apply as possible....

XYZ Food Stores Limited entered into a contract with a Canadian food wholesaler to purchase large quantities of raisins for its bulk food section.The contract provided that a 15% discount would be applied to the purchase price for orders of raisins in excess of 1,000 kg.The purchase of raisins was carried out with a separate purchase order and invoice each time. After several purchases of over 1,000 kg of raisins, the Canadian food wholesaler verbally indicated to XYZ that the 15% discount on the purchase price could now be applied to orders in excess of 100 kg.Since these were very favourable terms, XYZ began to order quantities ranging between 500-700 kg of raisins to adequately supply all of its stores. After it had been purchasing raisins for over a year, XYZ received a note attached to its most recent invoice informing it that it had made an error about the discount for orders in excess of 100 kg. and was holding XYZ to the original contract which only provided a discount of 15% on purchases in excess of 1,000 kg. The invoice showed the full amount owing for the most recent purchase of 650 kg with no discount applied and also a cumulative amount equal to the 15% discount which had been granted to XYZ on all previous orders. XYZ refused to pay the extra amount.

Providing your reasons, advise fully both the Canadian wholesaler and XYZ of their rights, liabilities, if any, and remedies.