I am taking a great liberty of writing you these few lines and trust you will not misunderstand my intentions.
The fact is that I have now been in your service for three years, and I believe that my work has been of real value to you.
When I joined your firm, I was told that my salary should increase to a maximum of 3,000 baht. Now I am getting only 2,500 baht.
I should therefore be much obliged if you would be so kind enough as to give me a rise in my salary as soon as possible.