U.S. Texas, bought land last August, now find out it's in a flood area and may not be able to build on it

Hello, Last August my husband and I bought an acre and a half of land in Parker county from the seller. We have a 72 month payment plan with the seller. Today my husband was talking to a friend of his who surveys land. He was trying to determine the amount of soil we would need to fill the land in before we can build on it. His friend looked over his resources and gave us some bad news. We can't build on that land. The entire center of the property is in a flood zone, per FEMA. Now, before we bought the land the city had a survey done, and we did the purchase through a realtors office. Our intentions to one day build on the land were clear from the start, but no where in any paperwork is it mentioned that the land can't be filled in or built on. Tomorrow my husband is going to talk to a civil engineer and see if it's just a matter of filing some paperwork. If not, from what his surveyor friend said, it may cost us 10-30k just to get the land surveyed and topographed to ensure it isn't in a flood easement. The land itself is only worth about 20k, according to the tax assessor.

So, what can we do? Is there any legal recourse to this that would get us out of the agreement with the seller if we can't build on it? I'm not sure that we can prove that they knew the land was useless for building a home.

Please let me know if I've left out any necessary information.