Registration

Dear Experts,

Please guide me on the following matter:

Mr. A has purchased a flat (out of his own funds) in Jamshedpur from a Promoter/ Builder. The Sale Deed has been executed and the possession has been received. Now, due to time constraint, the registration of the flat was not done.

Now, Mr. A wants to sell the flat to Mr. B (who is taking a Bank Loan), but the Promoter is not willing to give the NOC. 

Now, my question is whether the law permits the promoter/ builder to refrain from transferring the property in the name of Mr. B, after both Mr. A & Mr. B executes a sale agreement in between them?

Please suggest what to do in this matter?

Thanks in advance