amount given via bank and cheques received

We have given amount to some person via bank account over the period of two years. When we asked to return the money, the person has given post dated cheques and given a hand written letter mentioning that the amount will be returned before the dates given in the cheque and after that cheques would become invalid if the amount is given back. However, we need the amount urgently and the person has given close to 50% of the amount gradually. Now, the person is not returning the remaining amount and giving false hopes and confusing answers when we are asking about schedule of repayment of the remaining amount. Should we wait for the cheque date or is there any other way of pressurizing legally to that person?