Cancellation of sale agreement

We are planning to sell our flat in mumbai for rs.75 lac.The flat is in my mother's name. The building is more than 35 years old. It was constructed around 1981-1982.This old building don't have occupation certificate.My parents purchased this flat from the first owner.Sale Agreement between the first owner and builder is not available. But the stamp duty for our flat was paid in 1986 when my parents bought this flat.Share certificate is also available with us.It has the name of the first owner and my mother's name.This building will go for redevelopment in 1 year or so. Now the new buyer will be paying rs.25 lac as own contribution and rest rs.50 Lac thru loan from a bank. As per my knowledge, banks will sanction the loan only after registration of flat in the buyer's name. My worry is after registration what if the bank do not sanction his loan saying that the building is very old. So should i mention in the agreement that if remainder rs.50 payment is not paid within 1 month due to whichever reason then agreement will be cancelled and the rs.25 lac will be refunded after we get another buyer?Because we may utilise his rs.25 lac for some other emergency purposes. Please advise. Ajay.S