Do not pay the remaining money. Start by politely pointing out the error to the concrete contractor and ask them to come over for an inspection. If your not familiar with this type of work, it might be worth your time to find a structural engineer to come over for 2-3 hours for that meeting so you can have a knowledgeable person on your side.

3" is a HUGE error on an addition that small. The framer will likely not be able to compensate for it - a good framer likes to build perfectly straight, square and plumb and needs a perfect foundation to achieve it. Hold back all remaining payments.

Thanks! So besides witholding payment, what options exist to fix it? Is there a way to fix it without knocking it down and re-pouring?

Did you hire the foundation people yourself or are you working with a general? The hump can probably be ground down but the 3" out of square will be very hard to deal with and short of starting over you will probably have to live with some degree of imperfection. The framer will likely be able to absorb at least some of that 3". The concrete guy shouldn't get another cent though.

Ask the framer if he can compensate for the out-of-square and out-of-level foundation please. I'm betting this isn't the first time he's seen this which is why he was checking. Withhold any additional charges from the foundation sub's bill.

Check for square yourself too please. Pull a measuring tape from one corner to the opposite diagonal corner, then measure from exactly the same spots on the opposite diagonal. The difference in the measurements is the amount the walls are out of square.

Thanks! I hired the foundation guy on my own unfortunately :) The framer seemed to think 3" was pretty extreme in his experience, but yes, he did say having to compensate for some is typical. I'm going to have the framer and the foundation guy meet together to talk through this...wish me luck! Thank you everyone for the advice!