Reports suggest that he is a transfer target for Everton, while he is understood to have previously attracted the interest of several other clubs.
Koeman does not want to block the progress of a player who appears destined for the very top of the game, and says Feyenoord are prepared to listen to offers.
The Eredivisie club are also open to the idea of letting a couple of their other squad members leave.
"Sometimes you can't stand in the way of a player who needs a bigger stage to develop into a world-class player," Koeman said, quoted in the Sunday Mirror.
"We have two or three players who we may have to sell, and Bruno is one of them."