Dear Mr Scott,
I am writing to thank you for taking over my work while I was off ill last week. I came back this morning and found everything in order, and I know that it was only because of you.
I realise that you had to deal with my clients as well as your own, and that you stayed late on at least two evenings to finish the monthly invoices. It was extremely generous, as you were already busy preparing for the product launch.
Your help made a big difference to me. Instead of worrying about the office, I was able to rest properly, and I am now feeling much better. Without you, I would have come back to a mountain of problems.
Of course, I would be glad to return the favour whenever you need it. If you are planning a holiday or are ever unwell, please just let me know, and I will cover for you. In the meantime, may I take you to lunch on Friday as a small thank-you?
Best wishes,
Hassan Ali