Need to see! An inverted follicular keratosis (upside down seborrheic keratosis) is highly unlikely given the location. You could have a solitary wart that has become cystic (verucous cyst) or more likely a porokeratosis which is a callus (keratoma) that occurs within a sweat gland on the bottom of the foot . Regardless it needs to be seen to make the correct diagnosis.
Don't take a chance. Don't take a chance with self treatment. It sounds as though you have an ulcer and that is something you don't want to mess around with. Have it checked out.
Inverted keratosis. On sole of the foot a raised or flat lesion with a hole in the center can be an inverted keratosis which is callus like on a weight bearing area. This can be derided or scraped down to normal tissue. To prevent recurrence the area will need to be padded to minimize pressure. Since there are other possibilities it is best to see dermatologist or podiatrist for treatment.
Need more details. Your description is to vague. Please provide more details and a better description. Thanks.