Feeling guilty can’t sleep?

I was working in the shop .i tried to help as much as i can .i never thought anything wrong about that shop .after few month beacuse of some reason i had to quit the job ..i been so many times to the shop after i left .but one of my workmate charged me less .i didn’t know about it and when I realised i told her not to do it..now my x boss knows about it as well but now all messed up..i am feeling guilty because she charged me less..my boss must be thinking i was involved..but I wasn’t.i used my card contactless and never checked amount.what should i do now ?shall I return money ?or call him ?what ?i dont want to ruin relationships with my boss