Saints have been looking to bolster their front-row options after Sharman was released because there was "no immediate prospect" of him returning to full fitness from a knee injury that has sidelined him since January, while Andy Long retired at the end of the season.
Freer, 24, has a year remaining on his current contract at Headingley but Leeds have granted the former England Under-20s international permission to speak to the Saints.
Leeds took the decision after they agreed a season-long loan with Newcastle front-rower Joe Graham.
Former Carlisle and Tynedale number two Graham made 22 appearances for the Falcons last season.
Leeds director of rugby Chris Gibson added: "We are pleased to have secured Joe ahead of the new season.
"Northampton were in a position that they wanted to sign a hooker for the new season.
"Scott is contracted to us and we are keen to keep him at Leeds Carnegie however we recognise the fact that if a Premiership club make it known to a player that they are interested in him, that that can become a distraction."
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