The Blues boss stated it would require a ‘good offer’ for his club to cash in on their most valuable asset this summer.
And Cowley has stressed he’s not looking to see one of his key squad members depart in the coming weeks.
Championship sides Swansea City and Blackburn Rovers are among those who have been linked with moves for the 26-year-old.
Harness returned 12 goals and seven assists this season, with his stock continually on the rise since his 2019 arrival at Fratton Park.
Pompey paid £750,000 to bring in the winger and would be looking to at least recoup that amount, after taking up an option to extend his contract for another year.
Cowley is firm on the fact the club can’t afford to weaken themselves if they are to progress.
And it would take a sizeable offer for Pompey to do business.
Cowley said: ‘For us, Marcus was a key player last season.
‘He contributed with both goals and assists.
‘We’re respectful. For us to sell Marcus it would have to be a good offer.
‘Our focus is on trying to get better. We don’t want to lose key players if we’re going to achieve this.
‘We wouldn’t want to sell Marcus unless we get a good fee, ultimately.
‘He’s a key player for us.
‘We’d have to be well remunerated to then be able to spend it to help us get better.
‘Ultimately, though, you don’t get better if you sell your best players.’
Cowley is facing up to another sizeable overhaul of his options this summer, as he prepares for the transfer window to open on June 10.
Although stressing he doesn’t want to lose his main players, it’s clear gaining a decent fee for Harness would give Pompey more flexibility as they go about their transfer business.
The former Coventry trainee looks to be the one player who could bring a decent amount of money into the Blues coffers.
Ronan Curtis had a £2m valuation on his head 12 months ago, with a number of sides tracking his progress.
Curtis has not been able to maintain his upward trajectory this season with his form fluctuating, as he lost his place in the Republic of Ireland squad.
The 26-year-old still managed to return 10 goals and eight assists for Cowley’s side, however, as he was used in a number of different positions along the front line.
The pair appear to be the most valuable assets at present, although News/Sports Player of the Season has agreed a new two-year deal.
Meanwhile, Wales international Joe Morrell has two years remaining on his existing agreement after arriving from Luton Town last summer.