Regarding Frappe Validation

I want particular Item to be negative in Sales Invoice

If you are referring to Negative Stock, go to Stock Settings and enable it. If you want to set it for a particular item or item group, you need to customize the Negative Stock method and set the conditions accordingly.

How can I do it. Do I need to write a serverscrit code ?

Using a custom app then it’s possible. using server script, depending on what you use the code.