@Lumine_iris Unless I am getting confused on what that letter is, my guess is:
Since Taisho finally managed to give Ren's letter to Kyoko (The one he had forgotten to deliver.), Kyoko must have messaged Yashiro to find out Ren's schedule for the day, asking to meet Ren. Yashiro being the highly keen individual that he is has probably already intuited that Ren must have given the necklace to Kyoko. That's why he said that the "pendant is coming back", because even if Ren doesn't want to ask for the pendant back, Yashiro knows that Kyoko would give it back if Yashiro tells her the situation.