More than £4,000 was stolen from the BP Service Station at Golden Cross, near Chiddingly, on Friday (January 9).
At around 6pm a man approached a member of staff and asked if he could use the toilet.
He was taken inside and given access to the staff toilet in the private area next to the shop.
The following morning it was discovered that £4,385 was missing from the back office.
The suspect police want to speak to (pictured) is described as white, with short black hair and a slim build.
He was wearing a black trousers and a blue jumper, which has a distinctive white horse design on the front left-hand side.
PC Allison Briggs said: “If anyone recognises the man in the image, or knows his whereabouts, I would ask them to email email@example.com or call 101, quoting serial 0381 of 10/01.”
Alternatively anyone with information can contact the independent charity Crimestoppers anonymously on 0800 555 111.