1. (Beraisa - R. Nasan): "To the one Asham (the principle) belongs" - this teaches, if Shimon owes money to David, and David to Moshe, we take money from Shimon to pay Moshe.
Is there any reference to RIBIT according the case above (someone in our lesson suggest that there is different between the cases, according the ribit)?
2. Younger brother: I am afraid you will do as the trickster of Pumbadisa (81B)!
Older brother: If you want, I will divide the property with you now!
How didn't he know RAV YOSEF's rule either?
1) I don't think this law has any connection to Ribis.
2) Tosfos (DH "Pumbedisa") asks your question, and answers that Rav Yosef did not give a ruling in that particular case, but rather spoke about the Halachah much later. The story and the ruling were not connected.
All the best,
Yaakov Montrose