1) signing the contract is a with a precondition to amend the payment terms as designed by you .
2) without fulfilling the the requirement as required by you in the contract agreement ,is not valid ,on the grounds of " principles of natural justice " .
3) u r complaint is sailent whether u executed the work , without amending the agreement as desired by you.
4) if the work has been taken up without insisting the amendment as drawn by you, is a question of law , can be argued either way .
5) How ever, execution of work by you, without harping on the amentment can also be on a good harmony of business relations with the party , to avoid the delay in completion of the final date ,if any, inanticipation of the amendment.
6) signing of the agreement by you is always a precontion subject to the accepting of amendment to payment terms to the contract.
7) ignoring the conditional clause as insisted by u , is a unilateral decision by the other party and always questionable in law.
8) the payment of 65% of the contract by you comes to the surface of the case ,only after sorting out the amendment issue as requested by you.
In the light of the above, being a thread bearing and sensitiveness of the case , if deligent care is taken ,positive result may yield.
SUDHAKAR . RVS