Sale agreement and power of attorney

Please provide answer to my query:

Person a granted "power of attorney" to person b during the selling of a land and sale agreement was also executed between person a and b.

Now person b (the attorney) health condition deteriorated and was not able to transfer the land in his name using the attorney.

Person b dies, the "power of attorney" is no more applicable but sale agreement was executed between person a and b.

My query is: is there any value of sale agreement now and how can the family of person b claim the land in their name without involving person a?