If you side with the soldier, then the quest to extort money out of him will fail, though you do get experience for finishing it that way. After that, it is better to just not mention it to Locke again. You can confront him with the fact that he already got Richard to get what he was owed (for a second time), but Locke just dismisses that as a lie or misunderstanding (don't recall) and raises his prices. There is no extra reward for telling Locke to leave the soldier alone.
