England fly-half Owen Farrell is being quizzed after one of his allocated tickets for the game against Ireland sold for £440.
The ticket, originally worth £70, was purchased by an Irish fan living in London online at the last minute to attend last Saturday's fixture.
Farrell is not believed to have had anything to do with the sale, having given the ticket to a friend or relative in good faith to use for the match.
However, Farrell could still face a sanction regardless, such as having his ticket allocation suspended.
A similar incident in 2009 saw England back-rower James Haskell lose his allocation for three matches, with the RFU taking a tough stance regarding tickets being sold on the black market.
"We are satisfied that Owen Farrell did nothing untoward intentionally and had no knowledge of this sale," said an RFU spokesman.
"We are looking into the circumstances around how this ticket came to be available on a secondary ticketing site, something we take extremely seriously."