The Chancellor and the Mayor of London had earlier come face to face in China, where both are on separate trade visits this week.
Mr Johnson used the event to encourage Chinese students to come to the UK by, rather bizarrely, suggesting they could end up kissing Harry Potter.
But it was his comments on the visa programme that suggested the leading Conservative was not singing from the same hymn sheet as the Chancellor. Asked about the announcement that some Chinese tourists would no longer have to apply for a separate UK visa if they were visiting the EU, he said: "We'll have to see how this scheme actually works because the detail is a little bit unclear."
Earlier, Mr Osborne had hailed the news as a "big step" forward in UK-China relations. These were undermined last year when David Cameron met the Tibetan spiritual leader the Dalai Lama, angering the Chinese authorities.
Mr Osborne used his trip to emphasise the need for the UK to change its relationship with China due to the country's economic growth.
Labour said the Coalition's immigration policy was "confused". Shadow immigration minister David Hanson said: "While George Osborne and Boris Johnson say they want more Chinese university students, Theresa May is still trying to reduce the number of Chinese students, as they are included in her target to cut net migration."
Mr Johnson told Chinese students: "Who was Harry Potter's first girlfriend? Who is the first person he kisses? That's right, Cho Chang, who is a Chinese overseas student at Hogwarts school. Ladies and gents, I rest my case."