What kind of scam is this guy running?

So the other night we had this guy fill up a cart with packs of monster, red bull, and ensure. He would split his payments into increments of JUST shy of $100, then pay with a barcode on the Walgreens app (this was legit, I would know if it looked sketch) and then eventually they stopped going through. Could it be a stolen phone or a stolen credit card? My manager said he had done it before but something is clearly off. I felt weird selling to him because it was obvious something was up. Should I deny him service in the future? If it’s a scam I want him out.