As is car check by deal has fuel leak?

My fiancee traded in a reliable 93 ranger for a 97 monte carlo. We were told by the dealer that the car was checked and it only had an exaust leak. We took the car to a local mechanic four days later and he said the exaust was snapped off near the headers, but the gas leak was really bad. The top of the gas tank had rust spots all over. We took it to the dealer and they said the didn't know about it, so either they lied about getting it checked out or they lied bout knowning about the gas leak. The gas tank has two hand print sized wet spots that smell of gas from the top all the way down the left side. What is ironic is that the car was at the same mechanics not too long ago. The mechanic said they would change the oil if the didnt already last time it was there. So the dealer said he would try to help. But im wondering if its against Indiana state laws to sell any motorized vehicle with knowledge of a fuel leak? And what should we do if they wont pay if we dont have the money to fix it?