Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
I've looked over your request and unfortunately I have to turn it down at the present time. As you know, these aren't the best times for this organization, and we're going to have to keep our costs down in order to survive.
You're a valued employee. Please don't take this the wrong way. We'll re-visit your compensation at your next annual review.
Sincerely,
[name]