Once you have tendered payment and been denied, you still owe the money but it becomes his obligation to quote you the price. There may be a limitation to the time in which he has to do this.

That said, I found it at Amazon for $114
http://www.amazon.com/FSA-OS-115-Roa.../dp/B0006FNAOC

Since the stem was "just a tester he bought last year" I would give him a personal check, cashiers check or money order for $50-60 with FSA OS-115 Stem written in the notes area. Keep a copy for your records in case you don't get a receipt. There is a good chance he will just accept it and all will be good. If he doesn't think it's enough it kind of pressures him to give you a figure.