In the meantime, you can make a warm compress with Epsom salts and water. Use a clean rag or paper towels and add a few tablespoons of Epsom salts to a quarter to half a cup of warm water. Soak the rag in the solution, and (you may need help) pick the duck up, wrap the foot with the rag, and (hard part) hold the wrapped foot for up to five minutes. I used a shallow dish and even held the dish up under the foot to be sure to get a good soak. Sometimes Romy was done after three minutes, but if you can do that twice a day at least until you get antibiotics, it might help a little bit. I would also follow that with topical antibiotic ointment, anything to try to get the swelling down and some antibiotic to working at the site of the infection, which I assume it is based on the photo.
Hugs to you both!