I want to get the balance leave

It would be preferable IMO to handle the textbox's Leave event so that you can be sure that the user has finished typing but, if you want to do it dynamically, try:

        private void txt_availed_TextChanged(object sender, EventArgs e)

        {

            int availed;

            if (int.TryParse(txt_availed.Text, out availed))

            {

               int eligibility

               if (int.TryParse(txt_eligibility.Text, out eligibility))

               {

                  txt.Balance.Text = (eligibility - availed).ToString();

               }

            }   

        }