If you were an Indian citizen at that time and if POA deed was executed by an unregistered deed then it is advisable that you better execute a fresh deed with additional recitals enabling him to execute a registered gift deed unto himself by himself as an agent to the beneficiary. 

This is legally acceptable. 

1. As you have rightly suggested, gift the flat to your father so that he will sell the flat as per his sweet will and wish by obtaining tax exemption for a new super senior citizen.

2.  If the existing POA has authorised your father to take care of gift formalities, then the same POA holds good. However, if the existing POA does not have gift formalities, then you can execute a fresh POA in your father's favour to execute the gift formalities.

3.  The execution of POA and gift formalities can be completed within a fortnight.

Is POA duly stamped and registered .?

if not is it attested before Indian consulate .?

your father can on basis of POA sell the flat .TDS would be deducted by buyer at 22.6 per cent 

On basis of POA your father can also gift the flat 

You xan gift the same there is no issue for the same. Execute gift deed. 

you will have to first check in the existing POA whether any power is granted to your father to transfer the flat by way of gift and to register such gift deed with the sub-registrar of assurances

if the above power is absent in the POA, then you will have to make another 'Special Limited POA' in which you would authorise your father to complete and comply with the registration of the gift deed on your behalf 

so you can have the gift deed draft and special POA draft prepared by a lawyer 

after the drafts are approved by you, the lawyer will take their prints on stamp papers

for the special POA the stamp duty is Rs. 500/-

for the gift deed, for gift of property by son to father, the stamp duty is 3% plus 1% metro cess, on the market value of the flat plus 1% registration fee alongwith registration agent's and lawyer's professional fees

initially the gift deed can be stamped for Rs. 500/-

the stamped POA and gift deed can then be couriered to you at your USA address 

you will then have to sign on the documents before the Indian Consulate/Embassy in the USA

Then you will have to send the signed documents back to Mumbai by courier

your father would then get the gift deed registered and comply with the registration formalities on your behalf using your special POA 

the POA can also give powers to your father to sign on applicable society share transfer forms and share certification application and to submit the same to the society for name transfer in his name

likewise a power can also be given to him to apply to all the relevant authorities like electricity supplying company, etc. to transfer the electric meter and other utility bills in your father's name 

your father will have to hold the flat for atleast 3 years in order to avail the long term capital gains tax exemption benefit when he sells the flat in future 

Under Instruction 2 B (iii) of Master Circular No.  4/2014-14 (Updated as on 25th, June 2014 you are entitled to gift to a person resident in India. POA executed then years back is not valid now you need to execute a fresh POA. Further. POA cannot be issued  to the person to whom property is propose to be transferred, issue the POA to another person and get the gift deed executed by him to father .

Dear client,

PoA remains valid only till the life of the principal. General power of attorney if confers powers to gift to a particular person specified and the General Power of Attorney is registered only then General Power of Attorney holder can execute gift on behalf of the owner.

A gift deed can be made and executed immediately. Just approach a lawyer and tell him about the gift to be given or received and accordingly he will draft a deed which will be signed by both parties and get it registered.

Thank you.