If builder defaults on payment of refund money after cancellation of an under construction apartment

Hi, Background - I wanted to go for transfer of allocated unit in an under construction complex. Say from unit A to Unit B. Unit A was allocated and sales agreement also executed with the help of SBI home loan. Builder and Bank both agreed for the transfer of unit from A to B. Builder allocated me Unit B but sales agreement was yet to be done. During this period, bank evaluated the price of unit B lesser than the price as quoted in allocation letter of unit B. The difference was almost 10% or 10L of the price mentioned in the allocation letter. Therefore, i refused to go with Unit B and instead wanted builder to let me continue with Unit A. However, by this time builder has already sold Unit A to some other customer. Now, I had no choice but to ask for cancellation of allocation of Unit B from the builder. Builder agreed for that and i and builder agreed for refund of certain amount after some deductions. I went to the bank and requested them to consider closure of loan after payment of their dues from the builder's refund. Everything has been documented and conveyed through mail communication. However, off late builder is not responding or committing anything with refund amount. What options i do have now?