The 30-year-old victim spotted a woman alone in her car and apparently stranded late at night on a Northamptonshire country lane.
But as he got out to help two men came at him brandishing a metal bar and forcing him to hand over his cash.
A police spokesman said: "The man stopped to ask if she needed help and the woman told him she had a flat tyre. But as he got out of his car, he was approached by two men, one of whom had a metal bar.
"They threatened him and the man handed over his money."
Northamptonshire Police say one of the robbers was white and in his mid-to-late 30s, 6ft tall, of a stocky build with short dark hair and stubble. He was wearing a dark Parka-style coat.
The second robber man was also white, about 6ft tall and wearing a light jacket and blue jeans.
The woman was parked in a VW Passat near Westlands Farm on the B4036 Long Buckby Road between 10.45pm and 11.15pm on Saturday, January 11.
She was also white and in her late 20s, about 5ft 7in, of a slim build and with dark hair which was tied up in a ponytail. She was wearing a grey hoodie with white lettering on the front and dark bottoms.
Police are asking anyone with information or who may have seen the incident to call them on 101 quoting reference 20000020767.