Difference between making something permissible forbidden, and stipulating to not do something permissible in a contract or oath

Although I can't find a text saying exactly this, making something permissible forbidden or making something forbidden permissible is a (I think major) sin in Islam; see e.g. [16:116](https://quran.com/16/116) for something related. Now this is clear in the case of someone outright claiming that something that is known to be halal by consensus - such as drinking water - is haram, or claiming that something that is known to be haram by consensus - such as zina - is halal. This could possibly even be considered shirk, I'm not entirely sure. However, the lines become a bit blurry for me when oaths and contracts are involved. I vaguely remember reading a hadith about a person making an oath that they would refrain from doing some particular halal action, and Muhammad saying something like "don't make the permissible forbidden for yourself" - I'm not entirely sure that's what I read. In an [answer to another question](https://islam.stackexchange.com/a/38575/17702) , the matter of the marriage contract stipulating that the husband cannot take another wife came up; if he did, they wife would be entitled to annul the marriage unilaterally. However, this raises the question: does that mean the husband would be sinning if he does take another wife while that first marriage contract is in effect? If his violating that condition is considered a sin, that suggests that marrying another woman has become haram for him, thus making something permissible forbidden. If he doesn't sin, then how would a contract have to be worded to be considered as "making something permissible forbidden"? In a nutshell: - is an oath like "I swear not to eat chicken until the day I die" halal to make? - what is the difference in a contract between stipulating a condition that obliges one party to refrain from something halal, and making something permissible forbidden?