Kinda looks like a blood blister, as tho he was stung or pinched. Or maybe he's the 1st toad with an actual wart. Really nothing you can do but watch it and him for now. Might not need treatment if it's just a benign growth. If no change after a few days or if it shrinks, and toad is otherwise acting healthy, should be fine to release back outside. If it gets bigger, oozes, seems like it's causing him pain, or worse in some other way, a herp vet visit may be needed.

Or maybe someone's seen this type of thing before and will swoop in to save the day. Hoping for the best.