It is, especially (but not necessarily) if the person promising not to sue has a valid claim. This is covered in any contract law textbook, but it's an old comm

It is, especially (but not necessarily) if the person promising not to sue has a valid claim. This is covered in any contract law textbook, but it's an old common law principle so there are plenty of free online sources, too. For example, in the US, see Bennett, 'Forbearance to Sue' (1898) 10(2) Harvard Law Review 113–118 [1]; in the UK, see Kelly, 'Forbearance to Sue and Forbearance to Defend' (1964) 27(5) Modern Law Review 540–545 [2]. [1] http://www.jstor.org/stable/1321438 http://www.jstor.org/stable/1321438 [2] http://onlinelibrary.wiley.com/doi/10.1111/j.1468-2230.1964.tb02241.x/full http://onlinelibrary.wiley.com/doi/10.1111/j.1468-2230.1964....