It's emerged Forest failed with a January offer for the Japan international.
Dutch sources say Ajax put forward €15m for Sano before Forest went higher bidding €20m - but both offers were rebuffed.
NEC technical director Carlos Aalbers has since stated to Soccer News: "We managed it internally. I first talk to Kodai, then his management. After that, I tell the clubs that we won’t accept any offers.
“They were good, mind you, but they’re very welcome in the summer. Right now, we’re on a journey towards something great, and we want to finish it together.”
He added: “€20m is, of course, a huge sum. The advantage is that we have Marcel Boekhoorn (a financial backer of the club) on hand. Without him, we couldn’t have refused such amounts.
“We’re still not standing on our own two feet. At the end of the transfer window, Boekhoorn told us even if they offer double, a transfer is not something we want.
“It’s understandable that Sano wants to make a move. Four clubs have made offers for him, including Premier League and Bundesliga clubs. We certainly want to work towards a good transfer for him, but the timing wasn’t right.
“He’s been flexible and never pushed back. A South American player in his situation might have grabbed me by the throat and stopped showing up for training. That’s why we’re going to offer Sano a top tier transfer next summer.”