Answer:

Your boss asks you to take on some of their workload, so that they can leave early on Friday. However, your schedule is full and you're not sure you'll get everything done on time.

A tactful response might be, "Thank you for trusting me with some of your responsibilities. I'm sorry that I can't help you this time because of my workload. Is there anything I could help you with next week, when I have more time?"