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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
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The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.