Solution already suggested to you yesterday. Fix up an appointment to carry this forward

1.  You have to pay Stamp Duty on the "current rateable value" of the property, under the present "amnesty scheme" of Maharashtra Govt., and unilaterally Register the same by following due procedure of law and regularize your "OLD" dated Sale Deed.  No need for New Sale Deed, from Builder.

2.  Builder has no right to ask any Money from you, in any manner and it would be a criminal offence on his part, for Cheating, Intimidation, Mischief, Extortion .....

how your father got possession if registered sale deed was not executed in his favour 

2) you have not mentioned whether builder is agreeable to execute sale deed at old rate 

3) stamp duty has to be paid on  current circle rate of property 

If you will show purchase in year 2002 penalty may exceed ten times the amount of the proper registration-fee. Show current but lowest price to get it registered.

Capital gain tax liability is of seller, why will you pay. 

if any agreement for sale is executed at old rate there should be recital in sale deed to that effect 

2) also that sale deed could not be executed earlier on account of father demise 

3) that full consideration had been received by builder in 2002 

At least you must have payment proof, You could have file the suit of specific performance of oral agreement to get the sale deed registered. 

But that may barred by limitation now. Get it registered on old price with penalty, no capital gain tax to buyer also.

What proceedure? You just have to execute the sale deed with the builder, either in capacity of legal heir of your dad, and get it registered. 

If the builder isn't willing on whatever pretext (capital gain tax), then you have to initiate legal action. 

You can show depreciation in the property value show as builder old property for almost 18 years so here value will reduce more than 40% from.current ready recknor.

If you want I can make a sale deed for you in depreciate value. You can take further registration follow up with registrar and here you have to less capital tax to builder.

Sale agreement can be executed on this date mentioning the price originally fixed and paid in advance.

Since you have taken the possession , you can straightway make a deed of conveyance and register it. 

Property is to be registered at the market value at the time of sale.  Penalties to be paid for the delay in registration at the prescribed rate. 

No question of capital gain is there. 

Penalty for late registration is best ascertained at registrar's office

Do you have payment proof of 7 lacs if not than register it on present value. Certainly, he will ask you to pay LTCG. Better register with penalty.

Builder has to execute registered sale deed in your mother favour 

no penalty is payable 

Get it registered for seven lakh only. Penalties varies from state to state. On an average or may be up-to 400% of the value of the property.

builder has to registered sale deed in favour of your mother. there is  no need to pay any penalty, execute fresh sale deed  as per current market value. 

You need to get the property registered by paying penalty. You can apply for registration and during adjudication the exact penalty charges will be disclosed by the sun registrar

The registration of the property will be done on old rate.

Property can be registered on your mother name.

The penalty for late registration will be according to current charges. 

Since your father has already paid the sale consideration amount in full but could not get the sale deed registered on his name and he unfortunately died in between, you and other legal heirs can file a suit against the seller to execute the registered sale deed in the specific performance of contract suit. 

The evidence for having paid the sale consideration amount as per the agreement of sale would be in your favor and also the rates prevailing on that date would be taken into consideration.

The builder need not pay the capital gains taxes because it may not be applicable if the execution of the registered sale deed is made as on the original date of sale agreement.

The possession letter and other evidences to prove your possession and enjoyment shall be sufficient to adduce evidence to state that it was an oral sale agreement.

You discuss with yor advocate and proceed on the advises received on all such further issues.

1. As per the information mentioned in the present query, makes it clear that your father bought the property and also paid the amount, but no agreement was neither any agreement  signed nor registration of the property.
2. I believe that you must be having the transaction under which you paid the money either through check or bank transfer.
3. Then you can use the same to force the builder to now execute the registration as you are also having electricity bill on your father’s name and possession as well.
4. You might have to approach the court of law to get your registration done.