He said he would match any offersheet

Barzal has the right to sign with any hockey team that he wants to...well maybe not a women's league. Lou's job is to assemble the best team possible with cap restrictions. Lou may just say," I've made the highest offers I'm going to make. If you miss one minute of training camp, my only offer is the minimum offer I can make to a RFA."

And that will Guarantee the second Barzal hits UFA he gone?

He's not a UFA for four years. I don't know why folks worry so much about losing UFAs. Let's say Barzal going the "Trouba", taking less money/term/security from Winnipeg in order to get to UFA as soon as possible. Barzel could take that chance. Then the Isles would have a bunch of cap at the end of 4 years (let's say 8.5m, Barzel's salary in year four as a RFA) to spend on a UFA or other players,
I'm sure Lou isn't low balling Barzal but the new Covid, flat league cap have changed the salary structure for UFAs and especially RFAs.

7 pro seasons would be 26 3 years

I'm sure many UFA and RFA wished they had signed extensions last January. The NHL announced they had were expecting a cap of about 86m for 2020/21. Then Covid and the world changed. Think Barzel and other living off their expectations from pre Covid days, PLD signs a two year deal for a "modest" amount , Taylor Hall signs just a one year deal, Hamonic and others not even signed.
I'm not sure what is "fair", but I do know in most cases, players are going to get a less $$$, sometimes much less than they were expecting just a year ago.

I don’t disagree and don’t necessarily think he’s worth more than 7.7 but if he feels he’s been low balled

In my personal opinion, I think Barzal is worth 7.5 a year for 2 or 3 years, once the league has recovered from this pandemic, that's when Barzal can or should sign a bigger contract.

If you’re basing that on league recovering I don’t see that.

If bar all wants 10 a year Lou shouldn't be giving in to that.