This was a short story in a book of children’s stories, ca 1945-50?

The owner of a mastiff bets that his dog will accomplish a verbal task no matter what. He shows the dog a piece of money ( I believe it was an English crown), puts it in the dog’s mouth, tells him to take it home and sends him off. He goes home to await the dog’s arrival. A long interval goes by with no dog. Finally the dog arrives, bleeding with many dog bites, but still has the crown in his mouth.

The faithful dog has completed his task at fearful cost!