It's probably the medicine like the vet said. You can just leave it if you want and it probably isn't going to be a problem for your bun, or ask your vet about trimming the fur there as this will probably keep happening until you have finished the medicine. Most important thing with trimming if you don't take your bun to the vet to do it, is it has to be done with extreme care. Blunt tip scissors and a flea comb or other thin comb in between the scissors and the skin, to prevent nipping the skin at all. Or buying small professional pet clippers with the right trimming blade.
Otherwise, soaking the hard fur might help in cleaning it up. But you wouldn't want to keep doing this as wetness isn't going to help with the fungal infection healing.
Did the vet check his teeth too, to make sure drooling from a dental issue wasn't the cause of the fungal infection starting?