I'm very sorry to hear this so much after the fact
. Honestly refrigeration seems to come up a lot with axolotls but I thought with dwarf frogs their temperature requirements were much higher and fridging would kill him... When I was looking around for more information about what I could have done wrong to put him in this position the minimum requirement was 70F...
As to the suggestion of "probing the sore" there never really was a sore...I don't even think it was a fungus looking back on it...one day his skin and muscle on the leg started disappearing (the skin seemed to be falling off at first causing me to think it was like a stuck shed on a snake)... I'm not sure I'm explaining this very well but by the time I amputated there was no "fungus" and all skin and muscle on the foot was gone completely, he couldn't use the foot at all and it appeared to be rotting on his body. The bones were turning brown and a few of his toe bones fell off before I really considered amputating (I don't think I took a picture that showed the missing toe bones before amputation)...I figured if anything I was just speeding up the process that would have happened without my supervision or antibiotics...
The frog is about the same body size as he was on the day of amputation (which was almost 2 months ago) and when I started noticing the "bone" growth it was only about 10 days ago, he seems to be full grown (he sings a lot at night, I'm not sure if this is a sign of maturity or anything) but his size is definitely the same as it was a month ago, after the amputation and before any "bone" was noticed.
Edit: he can move the bone, It's hard to tell from the picture or in life since everything about these guys just reminds you that they're dwarves (I need to invest in a magnifying glass) but is it possible there is like a thin layer of skin obscuring whatever else is growing and we're just seeing the white of the bone/new skin? I say this because as the "bone" has been growing it looks almost like there is skin stretching at the corners when the "bone" is stretched out. sorry I'm probably overly hopeful/optimistic but if someone can shoot me down I'd be happy with that too, don't want to have false hope