The testator cannot transfer the property by a testamentary disposition i.e., a Will for the property to which he has not marketable title.
Thew power agent or the sale agreement buyer has no title to the property hence any bequest made through a will is not legally valid.
If the Will is not valid then the bequeathed made in your father's favor cannot be enforced.
The GPA has no authority to transfer the property through a will of his own to anyone of his choice.
The sale agreement made in your father's name is barred by limitation at this stage since the limitation is just three years from the date of execution.
Since your father has not got the registered title deed in his name, any transaction made by him in respect of this property as a owner shall be invalid
First he has to get the property transferred to him by a registered sale deed and at this stage the principal or in his absence, his legal heirs have to execute the same.
This involves lot of questions of law which cannot explained in detail here, hence you can better consult a local lawyer and proceed with the proper legal course if you want remedy legally.