Stamp Duty when making a Will?

I am in touch with a lawyer and want to make a will. I have a property within the municipal jurisdiction of Kolkata which i am trying to sell during my lifetime. However if i am unsuccessful in selling during during my life time i would like to ensure that my son be able to sell the property and pass on 25% of the sale proceeds to my daughter (this should apply at whatever time in the future he is able to sell the property), until this happens he remains as the sole custodian and beneficiary of the property. I have stated the same in my will but my lawyer is stating that 7% stamp duty charges on the property value will be need to be paid to get the will registered to the above effect. I have the following questions - 1. Is the stamp duty payable right now at the time of will registration? The property is not sold yet and i obviously have no visibility when it will sell. 2. If yes, why is this stamp duty payable in advance? There is no transfer of title happening unless the property is sold, it may sell. may not sell, who knows. 3. If the stamp duty is required to be paid, what happens when the property is sold?