How much money is at risk?

If they made a deal, they need to keep it. If the dispute is the volume of dirt moved or removed, that can be calculated mathematically.

If the dollar value is substantial, hire an attorney that understands this type of violation. If he/she sees you have a case, they'll probably pay for the survey, prove up the violation and the contractor won't have much choice to ante' up.

Take lots of photos, right now. Use something/someone in the photos to show depth of field. Don't just take pictures of dirt.

Teach a man to grow fish...
He can teach to catch fish...