Can I buy a Flat from a coowner while partition deed is not complete.

Hi, there are four brothers who have received 40 Flats in total after an agreement with the builder. The property is ready to move now. I am planning to buy a flat from one of the four brothers (say PersonA). He mentioned that the partition deed is not yet complete as one of his brothers has an issue dividing the number of flats amongst them. 1. The brother whom I am dealing with made this point clear to me and is asking to go with an agreement of sale by paying initial amount and three of the brothers would sign the agreement. 2. He mentioned that the registration would be done two months from now after the issues among the brothers is sorted and I pay the remaining amount after registration. 3. He asked me to mention in the agreement of sale that if the flat registration is not done with in 2 months from the time of agreement that I would receive back the initial amount I paid along with interest amount. I am confused If I can go with this proposal. I am getting a reasonable deal and the owner seems to be clear and genuine. Please suggest.