can you take a picture of the underside too?
does it feel warm/hot ?
I have 2 welsh harlequins with what looked very similar big swollen toe in the middle of the foot and they were limping and at that time I was able to take them to an avian vet And he wanted to do surgery he said the infection was inside the joint because it was not Bumblefoot but when I told him I couldn’t afford the surgery, he said antibiotics might work and he prescribed antibiotics for both of them and it did work and now it’s seven years later and they never had another problem like that. so perhaps antibiotics would take care of your ducks issue. I’m just not sure which antibiotic maybe 
@casportpony  can suggest one and the dosage.
@Miss Lydia
for now I would start giving her warm Epsom salt water foot soaks (try not to let her drink any. It will cause diarrhea )and give her access to swimming pool so she can keep her weight off of it, but yeah, please get a picture of the bottom of her foot too first.