How did Aram justify Mourad's act of stealing the white horse in "The Summer of the Beautiful White Horse?"

To understand this question, take a look at Aram's thoughts and feelings when he is first offered the chance to ride the horse by his cousin, Mourad:

Well, it seemed to me stealing a horse for a ride was not the same thing as stealing something else, such as money. For all I knew, maybe it wasn't stealing at all. If you were crazy about horses the way my cousin Mourad and I were, it wasn't stealing.

In Aram's mind, when Mourad took this horse, he had no intention of selling it to make a profit and, therefore, his actions cannot be described as theft. As such, he does not need to justify what has happened because he does not believe that his cousin, Mourad, has broken the law.
Moreover, Aram is easily able to justify his own riding of the horse. He allows himself to ride because he is "crazy" about horses and this provides him with a rare opportunity to indulge his passion. His family is so poor that this chance might not come about again so he must seize it, before it is time to return the horse to its rightful owner.

In William Saroyan’s short story “The Summer of the Beautiful White Horse,” Aram justifies his cousin’s acquisition of the horse using his personal reasoning process. By examining the text for evidence, the reader can follow Aram’s thought process.
Aram and Mourad come from a poor but proud, honest, self-sufficient Armenian family. Stealing is unheard of in their clan. When Mourad arrives with the horse in the middle of night, Aram is forced to justify how the animal came to be in his possession. After thinking about it, Aram decides his cousin did not steal the horse because he was not going to sell it for money. The boys were only going to ride it, which he reasoned did not constitute stealing. If they decided to sell it, which would never enter their minds, then the line would be crossed, and the animal would be considered stolen goods.

Aram justifies Mourad's stealing of the horse for two reasons; one, Aram does not believe it is the same as stealing, as they weren't doing it for money but to ride the horse for fun. He also justifies it because he has an enthusiasm and love for horses, and this opportunity may not come again, since their family is so poor.