Yea, teams will totally pay more for 20 games than for 80 game
That's a bit reductive, especially in a salary cap world. There are way too many variables in play to definitively say that Kane returns less at the deadline that now (that said, there are also too many variables to say he necessarily returns more).
There may be teams that are too close to the cap to take on his salary for a full season, or may need to send salary back, that are able to take him on at the deadline. There may be teams that are interested in Kane but are too uncertain of where they stand now that might be more willing to give up assets at the deadline when they've gotten to see their team in action (and are sure that their first, for example, won't be a lottery pick)