Is assignment deed enough

Hi, Me with my friends are planning to buy a commercial floor space in Hyderabad. The builder had already entered into a lease deed to give this space on lease for 6years. He is ready to transfer the lease deed to the ultimate owners, by a Tri Partee agreement. Up to this it's fine. The land where the property is built was purchased by the builder from state government housing board in open auction.He possess the valid documents related to this.As per the land sale between the government and the builder, builder is supposed to share 5% of revenue at the time of registration of the developed property.In between, the government had changed and the new government wants to increase the earlier agreed 5% to 8%, for which the builder is not in agreement.The builder is still trying to resolve this matter with the government and hence he is not in a position to complete the sale deed registration at the moment.He only promises to resolve the issue ASAP and then complete the registration. Can we go ahead, purchase the property and do an assignment deed registration. Does this transfer the rights of ownership? Please advise.