Hi new2grammar! You may adopt this letter with minor changes.
Dear Mr. Xyz,
You have been working in this establishment for quite some time and due to your sincerity and trustworthiness you are entrusted with the important task of handling the cash. However, off late I have been watching you to be not only failing in your duties but also in maintaining a healthy relationship with your co-workers in the office. On one occasion while taking physical verification of cash, there was some shortage. This being the first incident, I took a lenient view of the matter and let you off with a verbal warning but there was a similar incident a few days ago. Added to this, there have been several complaints from office staff about your unbecoming behavior to-wards them which jeopardize the discipline in the office.
Though I still consider you as one of the loyal employee of the organization, these lapses on your part have made me to view them very seriously, so much so to a point of loosing confidence on you.. However, I honestly hope that you would realize gravity of the situation and take necessary corrective measures so as not to compel me to completely change my opinion about you and resort to grave punitive action.
Student or Learner