Need assistance for cancelling agreement for sale

I booked a flat, provided cheque ( however when I issued cheque I didn't have much fund and it was already communicated to the builder, he said he will present the cheque only when I let him know) , it is written it is shali land instead of vastu. However I didn't know about this so signed it. I got to know about these things only after when sbi didn't want to give me home loan since it is shali land. I already informed builder that I need to cancel this agreement due to some financial crisis. He was not at all happy with the decision but however after some discussions he said he will return me the cheques and will cancel the agreement. My question is how I will know this agreement is cancelled, will there be any cancel agreement on a new stamp paper done? It is mentioned in the agreement that builder will provide any documents requested by me but he hasn't done so when asking for mutation and shali to vastu conversation. Also it is written the builder will refund money after deduction 50,000 and only when the property will be sold to new buyer which I found as illegal clause by searching over internet . Please let me know if you need scanned copy of the agreement. Thanks in advance.