First treat it as an infected wound. Soak in warm Epsom salts with a healthy dash of Dawn dish detergent to soften the scab. Pull the scab off and see if there is any solid pus under it. If so, remove it. If there is no pus, just a thin soft scab, then it's likely a cancer tumor.
Finish with...
Bumblefoot is just a silly name for a wound infected with staph bacteria. It can occur on the shank and top of the foot as well as the foot pad.
When it appears on the front of the leg, often it is a result of a chicken that is heavy and is having trouble jumping up onto a too-high perch. You...