Complex sentence: Wattles received permission from Messner to train Dan Patch and developed the horse's racing abilities until 1902, when Messner sold the horse to Manley E. Sturges of New York, for a record $20,000.
Split into simpler sentences: Wattles received permission from Messner to train Dan Patch and developed the horse's racing abilities. In 1902, Messner sold the horse to Manley E. Sturges of New York, for a record $20,000.