That's the risk of fixed-price contracts. Some clients will withhold final payment until you do lots of extra things. Following the sunk cost fallacy, having

That's the risk of fixed-price contracts. Some clients will withhold final payment until you do lots of extra things. Following the sunk cost fallacy, having put a certain amount of time into the project, you'll do more so you can get paid. In most cases, the client knows it's impractical for you to sue them. You either have to do the extra work for free, or write off the time you already spent as a loss. Another risk I noticed with fixed-price contracts is that the client almost always doesn't have a proper spec. If I help them write a proper spec, then they can post it on the freelance sites and hire someone cheaper to implement it. If you start a fixed-price contract without a proper spec, you wind up with the problem you're having. Now, I just flat-out refuse fixed-price contracts.