In a LinkedIn post with a photograph of William, Matthew Gardner, a retired chief superintendent, said his brother-in-law had spotted Prince William, 39, and enjoyed a "private moment with our future King who was humble and working quietly in the background, helping the most needy".
"The finale to this unique occasion was when Prince William asked my brother-in-law if he wanted to buy the 'Big Issue', to which he replied "I have no change". At this point William produced a mobile card machine. You cannot teach that."
Mobile card readers have increasingly been used by homeless citizens as the number of people carrying cash in Britain has sunk over the past few years.
William's office declined to comment on the report.
Prince William, his wife Kate and the rest of Britain's royal family recently celebrated the Platinum Jubilee, marking Queen Elizabeth's 70-year reign, with four days of festivities.
As children, William and his brother Harry were taken on visits to homeless shelters by their mother, the late Princess Diana, as part of her charity work.
