Cranberry Summer by Wende and Harry Devlin*

A circus left town and their old donkey remains.  Maggie and Mr. Whiskers adopt it while they attempt to reach the circus master.  The main problem is it costs money to feed a donkey and Mr. Whiskers' money box is empty.  Grandmother, of course, comes up with a plan so the donkey earns his keep.  Then Cyrus Grape appears on the scene, threatening to destroy any hope of profit and goodwill.
*Highly recommend.