1. Who had made the arrangements to execute the said agreement? Was it done by the said broker cum buyer? If yes, then he has done it purposely to keep the door open for him to deny having entered in to any valid agreement with you conveniently.
2. The said agreement is void at law hence invalid since it was executed on the stamp paper which was purchased after the agreement was executed which is not possible.
3. If you have the bank account of the said broker and the IFSC of the branch where he has his account, then you can return the advance amount with interest by NEFT and thereafter write him a letter cancelling the deal for not executing an agreement.
4. If you do not have his bank account no., send him the said letter to him enclosing a cheque for the said amount of advance paid by him to you.
5. Do not worry about any court case since the said agreement is legally invalid.