Mortagaged to a bank

Respected Advocates,

I purchased an AGRICULTURAL LAND and got it REGISTERED but later i came to know that it was MORTAGAGED TO A BANK.Now what is the legal remedy OR What is the process?The seller is saying that , it is not his mistake ,and he is arguing that it could have been purchased after checking throughly.Please advice me.

Regards and Thanking You Sir,

Suri