1. Since there is no mention about the deduction from the advance paid in case of refusing to buy the property at a later date, the builder can not deduct any amount legally,
2. Moreover, 30% deduction is too high in comparison to market standard,
3. You can file a complaint case before the local District Consumer Dispute Redressal Forum alleging deficiency in service and unfair business practice claiming the entire advance amount bpaid, interest, damage and cost,
4. File the above Consumer Forum case only after collecting the 70% payment fro him towards refund of the advance.