Another approach: In Return of the Jedi, when Obi-Wan finally tells Luke the truth, I could flash back to the brief ROTS clip of Obi calling the lightsaber to him with the Force, and Anakin doing the same; followed immediately by another flashback to ANH where Ben says, “This is your father’s lightsaber.” Cut back to the ROTJ conversation. It’s therefore implied that when they fought, Obi-Wan got ahold of Anakin’s saber and that’s why he could give it to Luke, accompanied by a lie.