Cancellation of agreement to sale of flat

Hi We have an agreement to sale with a property owner by paying 20% advance payment. At the time of deal discussion seller clearly mentioned that everything is payed to builder including Corpus fund , annual maintenance..etc. But before going for registration we checked with builder and found that he is not payed that amount to builder. If we ask him that he is saying some nonsense stories that you need not pay, we have some other business dealings, so don't worry etc. But builder is clearly saying that either of us should compulsory pay that amount. So I have asked seller to get an NOC for the same, then only will go for registration. Now he became silent and not moving the things. We don't want to pay that amount from our pocket instead we want of cancel that agreement. But my worry is if he delays or don't pay my money back what should I do. And in agreement there is no any such word that Corpus fund. And there is no time limit mentioned in the agreement. I have also payed government challan for registration. How to take that money back? Regards Venkatesh