As a freelancer you have committed to only supporting existing features. What you do beyond that is up to you. You can just say you are not available for further feature requests, or too busy, or give them a quote that is very high and they will find someone else.
Personally, I think you should find the time. I bet you could if you really tried. Also give yourself plenty of time in terms of the timescales you give the customer, explain that you cannot start immediately as you have other commitments.
Finding and giving the work to a 3rd party is more complicated than it might at first seem. You might get lucky or it might go horribly wrong. I would avoid that if you can.
This commercial work is going to stand you in a very good position when it comes to job hunting or future work. I really would reconsider turning it down if I were you.
PS what Mario said above is absolutely right. Sort out what you want to do and be straight and up front with the customer.