Fiction, pre-1990, boy's beloved horse given away to boy's rival to be trained, but rival doesn't do the training himself so boy gets the horse back

Another attempt at finding this one. Any help gratefully received!

Read before 1990, but it could have been old even then.

The main character is a boy—I think a teenager. I think he lives on a ranch in the American west. I think there are a lot of horses around. He loves one particular colt or filly that has a patch of white hair.

The boy really dislikes another boy/young man. The young man is maybe a relative like a cousin or maybe a really young uncle. I think the young man has a bad reputation but seems to be behaving himself around older folks at the moment. The boy's father maybe says the young man is doing well and should be given a chance to show he has changed his ways. He gives the young man the boy's favorite colt to take away and train. The deal is if he does a good job with the training, he gets to keep the colt, but if he does a bad job, the boy gets the colt back.

The young man leaves with the colt and the boy is devastated, but he thinks the young man will do a terrible job training the colt.

When the young comes back with the colt, it is well trained. The boy notices that the white spot is gone so he wonders if it's the same colt, but he notices that the colt is branded now and thinks the brand may have destroyed the white spot. He writes a letter to someone where the young man took the colt to ask if it's true that the white spot was under the brand. He gets a letter in reply that yes, the brand was right over the white spot. He stops reading the letter in despair because he has lost his colt. Then someone else (I think a girl) reads the rest of the letter and points out that the letter writer says later in the letter that the young man did not train the colt but had someone else do it. So the boy gets the colt back and is thrilled.