If the swelling is still there, it wouldn't hurt to try a poultice to pull out any infection or perhaps a thorn piece, sand spur? Animalintec is a good brand. Cut the size piece you need, wet it and leave it on the foot, wrapped up, for 24 hours.
Good luck!!
That sounds like an excellent home testing and treatment method. If it's a malignancy, big If; it won't work and the OP can continue on with their vet for a biopy. If it helps, I can't imagine it being cancer.