In any situation, the appropriateness of cursing, slang, and other forms of taboo language always hinges on questions of time, place, and manner. With all forms of persuasion, context is crucial. In the context of negotiation, emotionally charged appeals can be a slippery slope, most often better to be avoided.
There are always exceptions, but consider this. In the wrong context, swearing may constitute bullying, emotional abuse, or sexual harassment. Slang terminology can either imply a spirit of inclusion into certain social groups, or exclusion. It can be taken as a sign of respect, or it might imply a reductive, dismissive attitude toward the audience. It all depends.
So, while it may sometimes be true that cursing, and slang language have their appropriate time, place, and manner in the world, a negotiator who feels like that rhetorical style is the most persuasive approach probably needs to brush up on their sales negotiation training, or risk cursing themselves later for failing to perfect their pitch the right way.
As a word of caution, before you go into a negotiation armed with a slang vocabulary, make sure you know your audience, your message, and the real meaning of the language you’re going to use. Here’s an infographic to help with that.