18. Non-enforcement except with variation.
Where a plaintiff seeks specific performance of a contract in writing, to which the defendant sets up a variation, the plaintiff cannot obtain the performance sought, except with the variation so set up, in the following cases, namely:-
(a) where by fraud, mistake of fact or misrepresentation, the written contract of which performance is sought is in its terms or effect different from what the parties agreed to, or does not contain all the terms agreed to between the parties on the basis of which the defendant entered into the contract;
(b) where the object of the parties was to produce a certain legal result which the contract as framed is not calculated to produce;
(c) where the parties have, subsequently to the execution of the contract, varied its terms.