sentences:  The same applies if the consideration is a performance for which the parties had previously contracted.For example, A agrees to paint B's house for $500, but halfway through the job A tells B that he will not finish unless B increases the payment to $750.If B agrees, and A then finishes the job, B still only needs to pay A the $500 originally agreed to, because A was already contractually obligated to paint the house for that amount.