The 24-year-old Englishman had scored three goals in the opening two games of the season before Wednesday’s switch for an undisclosed fee.
“There was a clause that the club met and Kane wanted to speak to them,” said Dundee manager Hartley.
“Sometimes we can’t do anything about that. That happens in football.”
Hemmings, who joined Dundee last summer after leaving Barnsley, scored 25 goals last season as he formed a productive partnership with Greg Stewart.
It was their form together at Cowdenbeath that had led to Hemmings, who began his senior career with Rangers, winning his move to Barnsley while Stewart headed to Dens Park.
And their latest season together led to the pair being on the four-man shortlist for PFA Scotland player of the year award last season.
But the Burton-born Englishman has decided to return to League One.
“It’s a big blow for us losing our top goalscorer,” added Hartley. “He’s been a main player for us, but there’s not a lot we could do.
“Kane is ambitious, I had a good conversation with him and he wanted to try something different.
“He wanted to go back down the road near his family. Financially, it was something he couldn’t knock back.