Selling a property without knowledge and consent of the bank, when the property is under a mortgage of a bank is a criminal offense.
Giving a POA in favor of father by a borrower may appear as conspiracy and still giving POA is also part of the breach of trust.
Except liquidating the loan to save your innocent father, there is no other way. This is a priority.
A litigant should approach the court with clean hands. The sale transaction is tainted with fraud, and any such transaction illegally with others without bank may not get immediate remedy.
Contact a local advocate and save your father from defamation at his age.
Bank has nothing to do with the illegal transactions and they have to follow laid down policy. You may bring pressures to the existing purchaser who is in possession.