It may be the type of bumblefoot that gets between the toes. Do you see a scab on the bottom of the foot? Here is a good link with a similar picture and treatments:
https://the-chicken-chick.com/bumblefoot-causes-treatment-warning/
You can begin treatment by soaking the feet in warm soapy water or Epsom salts for about 15 minutes. The bumb between the toes may need to be lanced and squeezed to get out any pus inside. If you have a scab on the footpad, the scab can be removed and there may be a solid pus plug inside that can be squeezed out. You could try soaking for a few days in a row before removing the scab or lancing, and that may soften them. There are many different ways to treat bumblefoot, and those include sugardine or Prid dressings, and Tricide Neo soaks.