It happened at Waitrose at Admiral Park earlier this week.
Guernsey Police say the person who took the money out forgot to take it with them, and the next person to use the machine took it.
Any witnesses who saw it happen at around 11:45 on Sunday 7 April should call Guernsey Police on 725111 or Crimestoppers anonymously on 0800 555 111.
Pictured top: Waitrose Admiral Park.