The Brixen Witch (Paperback, Reprint ed.)

An enchanted coin. A witch s curse. And rats, rats everywhere What s a boy to do? Fresh and satisfying for middle grade readers (Kirkus Reviews, starred review).
When Rudi Bauer accidentally takes a witch s coin, he unleashes her curse. Accident or not, he knows he s got to fix things, so he tries to return the coin, only to lose it on the witch s magic mountain just as the snows come. Plagued all winter by terrible dreams, Rudi tries to find the coin again in the spring, but it has vanished and a plague of rats has descended on his village.
Then a stranger arrives and promises to rid the village of rats for the price of the missing coin. Desperate to get rid of the rats, the villagers agree; but when they cannot pay, the stranger exacts a price too terrible for anyone to bear. Now Rudi is going to need all his courage and some help from his savvy grandmother and a bold young girl to set things right in this fast and funny adventure.