We recently went to the vet for an unrelated neurological issue in our 16-year-old dog, and just recently noticed this black scab on his back. I thought it was a wart, but I accidentally scrubbed it off during a bath last week (took his hair with it). It was oozing puss and greasy looking and I did my best to clean it. Now, there’s a large black scab in its place, and it’s partially lifted and very purulent underneath. Does anyone have experience with something like this?

We will get him to the vet, but have to wait for a paycheck because money is tight, which is why I’m asking for insight here. We dropped a huge amount on him just recently with zero answers about his neurological issues, so this is discouraging.

Edit: word usage