Normally, it's not necessaty to mention the changes on salary after probation which should be clarified in contract. So all we should do, the essence of this kind letter is to "motivate " the employee.
So pls find out the following edition:
Dear ______,
On behalf of the company- #######, we have pleasure to confirm with you a successful probation period fulfilled by you during last two months.
Since your joining date, ________, you show the skills and capability required by your current position.
We count on your continuous efforts, commitment to our company at your current position as ______(position).
Wish you more success and career development within ######!