My wife keeps a diary. We have to constantly move dates around to fit people in. A friend might cancel, or we might have to cancel, so instead of seeing someone this week, it might become next week or the week after. No one seems to mind. As long as your friend gets to catch up with you at some point that should make her happy. You have a perfectly good reason for cancelling, as long as your friend understands the reason, so you're not really letting her down. Maybe you can see your friend after work or on the weekend, if work is taking up too much of your time.