who is the rightful owner

The seller has cheated you by not disclosing to you the mortgage. You should have asked for  a non-encumbrance certificate before purchase of the property. Now you can offer to pay to the third party and get release from the mortgage. You can sue the seller to recover the amount paid to the third party. You can also file a criminal complaint against him for cheating. You are saying that the transfer deed was drafted by your lawyer. Does the deed contain a declaration by the seller that the property was free of any encumbrance?