Merris joins fellow player-coach Tom Morgan and young defender Jack Johnson in departing the Flamingo Land Stadium.
The experienced full-back was one of Steve Kittrick's first signings at the club and enjoyed a hugely successful tenure.
Merris was a League Cup runner-up in 2015-16, helped Boro achieve Northern Premier League Division One promotion in 2017-18 as well as being a North Riding Senior Cup runner-up that season too - before going on to win the same competition the next campaign.
Merris said: "I'm absolutely gutted that I can't stay because I've loved every single minute.
"The fans of the club have been amazing, they have not only made me welcome, but they have made my family so welcome as well. My kids have had a great time.
"I'd love to come back, I'd do it in a heartbeat, but that is football and people do move on."
Boss Kelly added: "Dave has been amazing since I've come in, but he can't continue the travelling he's been doing on the money he has been offered.
"The same with Tom Morgan, the door is always open to him, things could change further down the line."