If the parties agree in good faith to changes (usually increased payments) in response to these unexpected emergencies, the changes may be implemented, although the required performance has already been provided for in the original contract. Some critics argue that the rule can be too broad, as it makes contracts inflexible and hinders useful changes. Suppose a plumber agrees to replace all the pipes in a house with copper pipes, but before the project starts, the price of copper increases and it is no longer financially possible for the plumber to replace the pipe. The plumber could reasonably ask for an additional payment from the homeowner to cover the cost of increasing the price of copper. Even if the owner agrees to this change, the change may not be binding under the existing customs rule, given that the plumber already had an obligation to replace the pipes with copper prior to the change. The customs rule already in force is a consequence of the counterparty requirement. Since consideration making contracts enforceable must be “negotiated”, the consideration cannot consist of a performance that the party already had an obligation to exist. If, in any event, the party was legally obliged to do something, the approval of that case is not a new reflection that must be “negotiated” by the other party.  Therefore, the offer of a party already required under an existing contract, working time or law constitutes insufficient consideration for a new contract or the amendment of an existing contract. The mandatory legal rule does not apply if the parties mutually agree to change the terms of the contract. For example, the owner and contractor could agree to modify their contract to add a new window for the bathroom at the expensive price of 1000 $US. Otherwise, the parties could agree not to abide by part of the contract for a $500 price cut. Both amendments to the original contract could be applicable, as there is consideration for each change.
 The legal rule of obligation protects one party if the other party attempts to unilaterally change the terms of the agreement. . . .