I have successfully treated mild cases of bumblefoot with topical antibiotic ointment and Epsom salts compresses. In most cases, just the antibiotic is needed. Wipe off the foot, then smear ointment on the foot, top and bottom, and do that three times a day for several days, then once a day usually covers it, but you must keep it up every day, sometimes for a month to six weeks.
The Epsom salts are also very good, and are said to draw toxins out of a wound. I used it on my own personal case of bumblefoot last spring. Worked.
If you can upload a photo that would help us give a little more informed opinions. I do not like opening a duck's foot because of their habits and environment, it increases the risk of a major infection. That said, I realize sometimes it is necessary.
Please keep us posted.