Setting up Fixed Percentage discount

Is there a possibility for setting fix discount percentage to individual inventory items ?

No. A fixed discount percentage would be no different from a change in the sales price, which you can enter by editing the inventory item in the Inventory Items tab. Discounts are reductions from the defined sales price. See https://www.manager.io/guides/10530.

By doing so, we can even give discount to 100% which makes sales price zero.
where is the security ?
I want my sales staff to limit the discount on sales price column with maximum 25-30%.
I don’t want more than 25-30% discount set on sales price.
is there a possibility of doing so or not ?
Kindly advice.

I already answered your question—no, there is not. The “security” in sales pricing comes from review and supervision by management. If you cannot trust someone to follow your policy, you should not give them permission to create sales invoices.

but in such cases, number of sales staff operating, best way to limit discount on prices without anyone changing prices manually.
That way they are not allowed to give more discount then maximum allowed in system.