The 31-year-old Asian victim was set upon by two men at the BP service station on Herringthorpe Valley Road in Herringthorpe just before 6am today.
One of the men punched the victim several times in the face before the other sliced off the top of his left hand forefinger with what is being described as a meat cleaver.
The attackers then left the scene in a dark coloured vehicle.
Police said the victim had been dropped at the service station in a taxi with the intention of using its cash machine.
He was today in a stable condition in hospital.
The lead investigator on the case, Det Insp Richard Partridge, said: “This is an extremely nasty attack which appears to be unprovoked and is being treated as racially aggravated.
“A full investigation is under way and as well as reviewing CCTV, we have a team of detectives in the area and officers are supporting the victim.
“The incident is being taken extremely seriously and we can reassure members of the community that we are following a number of lines of inquiry to identify these offenders.”
The first attacker is described as white, tall and slim. The victim was unable to provide a description of his accomplice.
Witnesses are being asked to contact South Yorkshire Police using the force’s 101 phone number, quoting log reference 223 of February 27.
Detectives say they are also keen to speak to the driver of the taxi that dropped off the victim at the garage.