At the end of Ring of Solomon Bartimaeus purposefully disobeys (or misobeys) Asmira and returns the Ring to Solomon. Why?
He claims it is because he doesn't think that they should:
Lose the Ring to the sea and doom its Spirit to an even longer captivity.
He suggests here that he wants the Spirit to be free and hiding the ring in the sea would slow that process down however in reality the Spirit is doomed forever no matter what. The Spirit tells Bartimaeus that freeing him is
Beyond your competence.
This suggests that the Ring has a spell more powerful than the normal binding spells attached to it. In fact ti seems likely that the only way to free the Spirit is for multiple powerful magicians to work together to free it. Through the series it is shown that magicians do not work to help spirits. The only humans who might free the Spirit are Kitty and Ptolemy but Barimaeus has not met them yet.
Given that the Spirit cannot be freed why did Bartimaeus return the Ring. Other nations and magicians will still want the Ring so the Ring returning will likely cause the deaths of both men and djinn. Solomon is also likely to die soon so bringing back the Ring will result in either Solomon hiding it (Probably by throwing it in the sea) or Solomon's successor taking the Ring likely resulting in a war. Barimaeus could avoid this by throwing the Ring into the sea. Furthermore Bartimaeus would then not have had to carry the Ring back which would save him a lot of pain.
Given all the good reasons for putting the Ring in the sea why did Bartimaeus keep it with him.