Dubas has said that he prefers Sandin on his natural side and that he sees Sandin as a big part of the Leafs' future, so I wouldn't expect him to be traded if there is no resolution before training camp. However, the longer into the season this drags on, the higher Sandin's cap hit will be this year, as we saw when William Nylander inked his deal at the last possible moment.
Although he is without a contract still, Sandin is currently projected to play the 7th most minutes on the Leafs' blueline this season and is still projected to lead the Leafs' D corps in total Goals Above Replacement. That's good enough for 32nd overall, among blueliners. Not only that, but Sandin also ranks 5th in Goals Above Replacement per 60 minutes among blueliners in the entire NHL.
At this point, I wouldn't expect and offersheet for Sandin, as it would've happened by now if it was going to happen, but I'd suspect that this will drag into at least training camp, as the Leafs also have to move out some money to create cap space for the deal Sandin eventually signs.