Tempted though I am, I file myself unable to comment on potential points of failure in your particular arrangement without the actual agreement to sell before me. I will therefore limit myself to the eventuality that you referred to in your question: in which the post-dated cheques in question are dishonoured while sale deed has already been executed.
You will have recourse to the Negotiable Instruments Act in such an event. The drawer of a cheque becomes liable for punishment and fine when that cheque is dishnoured due to insufficiency of funds. After a cheque is dishonoured and the payee is informed of the same, he is required to send a legal notice demanding that the drawer pay up. The drawer than has 15 days to do so, failing which the payee can file a complaint under Section 138 of the Act against him.
Note that suit for recovery of money (a summary suit under Order XXXVII CPC at that) is also maintainable; in fact both remedies can be pursued at the same time. However, people usually stick to the complaint under the NI Act as the fine contemplated under Section 138 thereof is directly related to the cheque amount inasmuch as it says that a fine up to twice the cheque amount may be imposed on the drawee.
Hope that answers your question.