When we say "i will do something " we should say een shaa' Allaah. What do we say when we say "No, i will not do so-and-so"? Unless Allaah wills it?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Adding the term: "Inshaa' Allah (Allah willing)" after saying: "I will do such and such" is recommended and not obligatory. An-Nawawi said in his explanation of the book of Imam Muslim: "It is recommended for a person when he says: 'I will do such and such' to add Inshaa' Allah (afterwards), as a way of following the Command of Allah Almighty (what means): {And never say of anything, “Indeed, I will do that tomorrow”, Except [you should add], 'If Allah wills'….} [Quran 18:23-24]

There is no difference regarding these two statements, "I will do such and such" and, "No, I will not do such and such." The same ruling applies to both of them.

Allah Knows best.