A one sentence summary is clearly going to be very short and brief, so you need to think very carefully about how you are going to construct this summary and what information you need to consider and include in this summary. Allow me to offer you a few thoughts before I go and give you my suggestion.
Firstly, you might like to consider the theme, and restating that to give an overall summary of the story. Clearly, one of the central themes of this brilliant tale is the dangers of materialism and greed, and how it can actually distance you from love and human relations. This might be a good place for you to start.
Secondly, you also might want to use the title as the basis of the summary. Remember how important the title to a short story is. Would there be any scope for exploring the importance of the title in such a summary?
Lastly, you might want to consider the major characters and how they change or develop (or not) during the course of the story. This often yields important information.
So, putting these three together, a one sentence summary of this story could be:
A boy is forced to dabble in the supernatural and find out the names of winning horses to gain money for his mother because of her greed and materialism, eventually killing himself.