You can't do that without 1) paying off the mortgage, 2) refinancing the mortgage based only on the 36 acres you wiil keep, or 3) otherwise with the consent (agreement) of the lender. The mortgage is a contract; that contract has (I presume, based on what you write) all 106 acres  securing the loan; you cannot reduce the amount of property securing the loan without the agreement of the other party (the lender) or paying off the loan in one way or another.