Your son did say he'd pay back? Plan that money now, what you'll pay with it, when it will be paid, etc. It can be good that you found them what they need?

When I got married, I wasn't "allowed" (stepmother) to do much of anything and my stepsister bought an entire rack of dresses for my bridesmaids, so she'd be sure she got enough of the right sizes. Then she took the ones we didn't need back and collected money from my bridesmaids, etc.

It was good you asked to make sure your son's family could pay you back. Maybe this week approach the lunch eaters and tell them you're "short" could they buy for you "this" time? There's give and take in life. It does feel good to give, and doesn't sound like you're doing too bad with it. Sounds like you're learning?