Can you give us some detailed info of the setup? what sort of heat are you running?
Could be a minor burn, could be some sort of skin infection. Hard to say without viewing the snake etc etc. Even then it may not be entirely clear.
I had a similar looking thing on one of my adult jungles just after cooling started this year. Highly doubted burn due to the set up, seemed to occur just after a very dry shed that caused a lot of irritation on the belly scales. Vet gave me some anti-septic cream to apply to the bad areas and after another shed it seemed to clear up pretty well. everything seemed clean etc so we're not entirely sure what caused this.
I wouldn't be soaking, keep it dry. Is it due to a shed anytime soon? has it shed recently? Is is showing any signs of going into a shed even though it's not due?
I'd suggest contacting the vet and explaining the situation, hopefully they'll be nice enough to help you out and let you pay it off.