The former Aston Villa youth product – son of claret and blue legend Gordon Cowans – has suffered a succession of injury problems over the past year-and-a-half.
The 24-year-old has now reached an agreement with the club to terminate his stay at the New Bucks Head.
Bucks boss Gavin Cowan said: “Henry has been an absolute pleasure to work with and it’s with a heavy heart that we have decided its in Henry’s best interests to get game time and build his body back to full strength.
“I’d like to thank Henry for being such a class act as he leaves with our gratitude and best wishes”
Meanwhile, a late penalty from Elliot Durrell saw Bucks salvage a draw in their friendly with Stafford Rangers last night.
Rangers had taken a first-half lead at the New Bucks Head but Durrell was brought down in the box after 82 minutes and stepped up to convert from the spot.