One of the prevailing superstitions in project management is the "fixed pie" assumption. In other words, both sides assume that the project results, speaking mathematically, are binary - either the team delivers the project on time or it doesn't; either the project is on budget or over, etc. Fortunately, negotiations are typically not like NBA Finals when team A meets team B in a seven-game series where the winner gets the Larry O'Brien Championship Trophy and the loser goes home empty-handed.
In my experience, situations on most projects are similar to the fable involving two kids quarrelling over the ownership of an orange. Finally, their father enters the room and, operating under the fixed pie assumption, cuts the orange in two equal halves distributing the fruit between the brother and the sister. Interestingly enough, the brother eats the orange and throws away the peel. The sister uses the peel from her half as an ingredient in pastry while disposing of the fruit.
The situations between customers and project teams are often similar to the fable described previously: the underlying interests, constraints and risk tolerances of both parties are rarely identical. The proverbial pie usually looks quite different to each party. Hence, a good project manager can increase the size of the pie by looking for things that are of low cost to him and his team and high value to the customers (and vice versa).
Consider the following interaction between an experienced construction project manager and a customer:
Customer: “I would like to add another clause to our contract. If the work on the new mall is not finished by the deadline in the contract, I want your company to pay a penalty of $5,000,000”
PM: “Hmm, we have already signed the contract without the late penalty clause; I am not sure how our management would react to that …”
Customer: “I am sorry, but I have been instructed by my boss not to proceed ahead without this modification to the contract”
PM: “And may I ask you why you guys feel the need to add this clause?”