move the goalposts

To move the goalposts means to change the rules, conditions, or objectives in a way that makes them more difficult to reach the goal.
Move the goalposts意思是改变规则、条件或目标，使它们更难达到目标。
Imagine a soccer game where the players were trying to score a goal, but someone kept moving the goal side to side, or further away -that would certainly make it a lot harder to get the ball in the goal
想象一下，一场足球比赛中，球员们试图进球，但有人不停地把球门从一边移到另一边，或者移得更远——这肯定会让进球变得更加困难。

We'd almost signed the contract when the other guys moved the goalposts and said they wanted more money.
我们几乎就要签合同了，可就在这时对方那些家伙又出了难题，说他们得加价。