Homegame short on money at cashout

[Gelin1]

I run a small, weekly cashgame with buy-ins at ~$40. Last time we were 5 players, but last couple of hours only 3. When we decided to quit and cashout we were short $40. I'm 99% sure of who didn't pay when he bought in. I realize it's my fault, I should have made sure everyone had paid before giving out chips (I usually do) but apparently forgot this time. Since I'm pretty sure who it was, I sent him a diplomatic text message saying I think I forgot, is it possible you didn't...etc. I didn't receive an answer and I don't know how to proceed. We're all friends (kinda) and it's a very small sum, but it's not the money that matters. I wanna be able to trust the ppl I'm playing with

What do you think?