Hemmings left Dens Park on Wednesday in a sudden switch for an undisclosed fee, which is reported to be around the £250,000 mark.
Hartley said: “There was a clause that the club met and Kane wanted to speak to them. Sometimes we can’t do anything about that. That happens in football, where clauses go in to players’ contracts.”
The former Rangers and Cowdenbeath striker netted 25 goals in his first season on Tayside and hit three goals in his first two matches this season during the Betfred Cup.
That form attracted attention from elsewhere and Burton-born Hemmings has decided to return to Sky Bet League One little over 12 months after signing a three-year deal at Dens Park following his move from Barnsley.
“It’s a big blow for us losing our top goalscorer,” Hartley said. “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.
“We’re disappointed but that happens sometimes, we have to move on quickly and hopefully find another Kane Hemmings.”