The bomb was built with decision to delay shipment until the 13th to see if Japan surrendered. On the 13th no word had come, but the shipment was further delayed to wait for word anyways. On the 14th word of Japan's decision to surrender was announced. https://www.marshallfoundation.org/articles-and-features/gen...

Had the bomb already been shipped, nothing about these delays suggests it would have instead been dropped before surrender instead of just delayed there. In reality, the pause sources from when Truman intervened immediately after Nagasaki (which was bombed on the 9th, Hiroshima previously on the 6th) by the 10th in hopes of unconditional surrender instead.

Had Japan not surrendered for some reason (they had more going south at the time than just the nukes) the US may well have dropped a 3rd bomb over another approach. That said, I'll give due credit to Truman that the activity was paused on hopes of surrender first rather than waiting for the next one to arrive.