Generally, you are only obligated to the terms of the contract, so if the concrete company erred and poured more concrete then they should have, you would not have to pay for that. This could change, though, if you had asked them, after signing the contract, to pour more (in which case there was an oral agreement to pour the extra 300 lbs) or similarly, if an advance of pouring, they told you that they had to pour more than they first thought and you agreed to let them do that. However, in the absence of such an after the fact change or agreement by you, they would seem to be able to only charge you for the agreed-upon 200 lbs.