The 10,000 kg of material was not used because the relevant cost focuses on the material that is necessary for the specific contract or project at hand.
The 7,500 kg was mentioned as it represents the amount that could potentially be used for the contract, but since it was purchased in the past, it is considered a sunk cost and therefore not relevant to the decision-making process.
The relevant cost should reflect the current purchase price of any additional material needed.