How you can control yourself when you negotiate with other employee & he start to talk in bad way ?

Be with yourself and do not loose your temprament.  Talk them in a way that they ultimately accept your Authority.

I would keep calm, listen to him and understand the issue from his point of view and use my influential skills...

بعيدا عن المجاملات والمثالية نحن كعرب نتعامل مع الاشخاص بالمثل بمعني اذا الشخص الذي امامي يعاملني باحترام اعامله باكثر من احترام واذا كان الشخص الذي امامي يسيئ لي ولعملي اعامله بنفس طريقته 

الابتسامة ، و اتباع الخطوات اللازمة في اللائحة التنظيمية الكفيلة بعدم تكرار ذلك .

Don't loose your temprament, listen him in a professional way, ultimately he will accept your authority.

Stay calm in the heat of the moment is paramount to your personal preservation.

Remove yourself from the situation and treat it with indifference. Don't under any circumstances bad talk to their face or to anyone else because then you are sinking down to their level. Add something positive by redirection such as by focusing on something, anything, positive in the situation or in the conversation. Whatever you do just stay calm

التجاهل صدقة جارية على كل فقراء الأدب

By not allowing yourself to get emotional. Maintain patience , composure and tact and the other person will soon compose himself. If the behavior is inappropriate the employee needs to be spoken with and cautioned appropriately.

التزام الهدوء والتحكم فى الاعصاب اذا كانت المره الاولى ولو تكررت مره اخرى التحدث مع الشخص بطريقه صارمه ووضع حدود فى التعامل 

You need to keep calm and composure and no need to feel intimidated. Don't be uneasy as the person may be probing you to check how you handle pressure and resolve a conflict. Try to listen his point of view before answering back. Think on your feet and express your opinion in an assertive way. Control your emotions and remember negotiation involves two parties so try to create a common ground to work on. Try to reach to a mutual agreement if can't step back to find better way to resolve It. 

حسن الخلق من حسن الفطن  ادفع بالتى هى احسن فاز اللزى بينك وبينة عداوة كانة ولى حميم