Validity of Memorandum of understanding registered for purchase of Flat from mediator

I have paid a token advance of Rs 1 lakh to a mediator for purchase of a semi finished apartment. The owner/developer of the building had entered into an unregistered sale agreement with the above mediator for selling of 18 flats on a consideration of 33 lakhs for each flat. Basing on that unregistered sale agreement the mediator now is selling the flats on reduced rate of 21 lakhs and telling that the flat registration will be done after 15 months. Till such time the mediator will register a memorandum of agreement in our favour and we have to make the complete payment of Rs 21 lakhs to the mediator . In this memorandum of undertaking only the mediator is signing and the owner/developer is not willing to sign. Whether it is legally valid to enter into an agreement with the mediator who is not the owner/developer of the property and the mediator is not having any title deed or rights except the unregistered sale agreement on a stamp paper of Rs 100/- Please give your valuable suggestion