Your payment was made for a procedure not provided, and getting money back should not be an issue at all. Simply asking should be enough, but you should be ready to explain why, as your surgeon certainly has every reason to want to understand why you have changed your mind. If you are unhappy with your previous surgery or some aspect of your post-op care, your surgeon SHOULD know, especially if it has made you change your mind about additional surgery!
Also, you should understand that the cost you were quoted may have been made on the basis of both procedures being completed. My partner and I routinely offer multi-procedure discounts, though this is for surgeries done at the same operative setting, not two separate operations. Still, considerations may have been made under the assumption, based on your payment, that 2 operations were to be performed, and some discounts may not be refundable. If your breast reduction surgery was planned for the near future (within 2 weeks), then there may well also be cancellation fee(s) that are appropriately charged, since many other people are involved in the schedule and performance of a specific surgery, including anesthesia, surgicenter, nursing staff, and others. Changing your mind at the "last minute" does not allow the entire team to immediately re-fill the surgical slot you may have agreed upon.
As long as you understand these facts, and think not only from your standpoint but your (previous) surgeon's standpoint, then simply being honest and asking for the "unspent" money back should not be a problem.
Making a "scene" is certainly another option that nobody wants!
So ask nicely right away and put this worry out of your mind! Best wishes!