Firstly, I would like to congratulate you for the outstanding work done by you at such a short notice. The presentation you prepared for the (description of project) was truly exceptional. There is no doubt that we were awarded this project due to your fine work. I understand that this work required at least 2-3 months of research, and you have completed this presentation in just 25 days. We are really proud of you.
Please accept the enclosed check as our way of saying thank you for a job well done. We hope you continue such outstanding work in future as well. Congratulations again!