This is because consideration is a "tit for tat" or quid pro quo situation an agreement cannot be enforced in 'something for nothing' situation. It is the reason for the promise.
In the case of CURRIE VS MISA, it has been defined as 'some right, interest, profit or benefit according to one party or some forbearance detriment, loss or responsibility given, suffered or undertaken by the other. David has provided the necessary software for the new system. He has taken responsibility and legally he is entitled consideration. Ron and Sharon have also provided the hardware and written the manual respectively they have also taken responsibility forbearance and they are entitled to consideration.
Amber has made an offer to pay David 20,000 after supplying the computer software. David accepted the offer and by commencing to fulfil his obligations, David has simply accepted to be bound by the terms of the contract. The terms of the contract are that David would be paid 20,000 and finish the contract by 30/05/04. Therefore there exists one of the essentials of a valid contract i.e. consensus and ideam a part from offer and acceptance. This principle refers to a 'meeting of the minds'. It must be shown that both parties understand the subject matters of the contract before agreeing (assenting). None of the parties i.e. between David and Amber can deny knowledge to the terms of the contract.
However, an offer terminates if a counter offer is made to them. This is a reply to an offer whose effects is to vary the terms of the original offer. A counter offer extinguishes the original offer and the person to whom it was originally made cannot move it. However, it can result in a contract if its terms are accepted by the original offers.
In Hydes vs. Wernch, Wrench (w) offered in writing to sell his firm to Hydes (H) for 1000. It made a counter offer of 950. W refused to accept 'H' sued for specifics performance (an order to perform). It was held that there was no contract & specific performance couldn't be granted. The reply of 'H' stating that he wanted to purchase the firm at 950 was a counter offer extinguished the original offer of 1000.
What David has in fact made is a counter offer asking an extra 5000 amounts to a counter offer that extinguishes the earlier offer of 20,000. I would therefore advice David that he has made a new offer of which Amber can either accept or reject. In this particular case, Amber agreed to David's new offer that a further 5000 be paid on top of the agreed 20,000. If Amber could've refused to pay the 5000 then David could have had two options:-
a. Accept the original 20,000
b. Repudiate the contract but ask for quantum meruit-
This is a common law remedy it means as much earned. Where the plaintiff sues
to recover an unliquidated damages or by way of payment for services rendered is said to claim on quantum meruit. Now that Amber had agreed to the new terms, he's bound to pay David 25,000, 5000 being the extra-price agreed. 2
I would therefore advice David to sue Amber for the balance. On the event that David does not recover the 5000 from Amber he can receive the same from Zax