1. a factor or event that will end an agreement for at least one party
definitely a deal-breaker
'A deal-breaker' is a point or detail that cannot be excused or overlooked in an exchange or when making a decision or attempting to reach an agreement. It is an obstruction to a deal or solution. This can be used to talk about formal business deals but also about personal subscription deals, relationships or other social situations. Similar expressions include 'a stumbling block' and 'a breakpoint.'