[samrohan]

Sam had not carried the ring for long enough, it had yet to assert itself over him.
Secondly, Sam was made rather naive in a very nice way. He all the while had accepted the fact that his master Frodo was the ring bearer and he was only taking care of it when Frodo could not himself.
The ring was never passed to him so he had no problem giving it back.

The quote reminds us that he does not really give it back but offers it and also offers to continue carrying it. He gives a choice to Frodo. The ring then sees a chance to regain control, for it never had control over SAM and forces Frodo to lunge out and grab it back.

I personally believe that Sam was made of sturner stuff and would prove a difficult target for the ring.