İs this sentence natural? If not, how would you say it?
Negotiations are going positive.

Question

Answer

It's not grammatically correct. The easiest correction would be to change positive into an adverb:
"Negotiations are going positively."

More commonly, you would see people use 'well'. For example: "Negotiations are going well."
OR: "Negotiations are going smoothly."

Answer

You should always use an "adverb" to describe "verbs". So negotiations are GOING well/positively