There is clearly a difference, in good project management, between goals and objectives. Goals are which we hope to attain by undertaking the project; however, they are not always attained by project end because there may be other external influences that factor in the their achievement or lack thereof. Objectives are, well, objective, in that they can clearly be checked off as having been attained by project end.
For example, “increase sales by 10%” would be a goal of the project not an objective. An objective would be the “installation of the system.” The acid test question to distinguish one versus the other is, “Can this (goal or objective) be clearly accomplished when we consider the project to be completed?”
I am not saying that we should not continue to track project goals post completion; I am just saying that they need not be accomplished in order for us (and the customer) to consider the project done.
Lastly, if a customer wants you to guarantee the attainment of goals than I would have you consider contingency pricing. For example, if they do increase sales by 10%, you should be paid 50% of that number. In a sense, a customer asking you to guarantee goals, is like that customer asking a lawyer to guarantee winning a case and therefore subject to a contengency arrangement. I would, however, guarantee meeting all objectives.