While I can see the reasoning with the act of buying meat to save the deal, I agree with Frognot. You shouldn't cover up for it out of your pocket. If it's bad or funky in any way, tell him about it. Let him know that it was probably due to being 4 years old. He may not even have seen the date, but needs to know about it. Also let him know that if you proceed with cooking it and it makes people sick or just tastes bad.....well, then that's not something you want to be saddled with.
Tell him to break out with the ribeyes, t-bones, etc. He's holding out, I tell ya!