Whether there is a covenant in the "agreement to sell" that the "agreement" shall be valid subject to realisation of cheque ? If so, it (agreement) is not binding upon you.
However, if you are keen to stick with agreement you may issue notice qua dishonour of cheque and proceed as per law.
In a "sale" transaction there is "consideration" but not "remuneration" as stated by you, recheck with your tutor.
I have all my doubts about the profile of author, who have stated to be a "LAWYER". If it is correct, he requires extensive classes/ tutions/ guidance.