Is there any in-movie explanation in A New How explaining why did Han Solo change his mind?

We see him packing the cash that he received for saving Leia, and some moments / hours / time later he is saving Luke Skywalker’s life and, eventually, the whole Rebelion. A (too) important change to be made in such a quick period of time?

I am aware of this great answer, but my question isn't a duplicate (at least in my opinion), because that answer is by far more general (on getting Solo from Imperial to Rebelion side) and — what is the most important for me — it is based on many additional sources in the Star Wars franchise.

Here, I am specifically asking, if it is explained for sole A New Hope watcher (and I somehow missed it), why Han Solo changed his mind / side / attitude so rapidly? Or if in the context of this movie only the simple answer is "no, it wasn't clearly explained"?