The Saints are believed to have bid £12m for the striker, just seven months after he made the move to the KC Stadium for a fee in the region of £7m.
"We'd not been looking to sell Shane but it's a fantastic offer and we've decided to accept it," Bruce told the Hull Daily Mail. "The challenge is always to become bigger and better and this money will be put towards strengthening again.
"Whether you are Cristiano Ronaldo or Lionel Messi, everyone in football has got their price. Myself and the owners didn't want to lose Shane but we accept it's a very good offer, especially when you consider what we paid in January. I've got no issues."
Long has scored four goals for Hull since joining from West Bromwich Albion in January.