Mohamed Salah is expected to join Fiorentina for another season
The 22-year-old joined the Serie A side on a six-month loan last season, but has the option to extend his stay for the 2015/16 campaign.
And Della Valle has told reporters that negotiations with the Premier League club have reached a 'decisive' point.
He said: "We are trying to keep him, this is the decisive day.
"Personally, I am optimistic about the negotiations succeeding. We will understand everything this evening."
And the club's sporting director Daniele Prade is similarly confident after confirming the offer made to Salah was 'insane'.
He said: "I tell the fans that the club has made an insane offer to Salah.
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Salah has the option to sign for Chelsea should he extend his loan spell next season
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"If any player, not just Salah, turned down a proposal like that, then it's best he doesn't stay at Fiorentina.
"However, I think Salah will accept. He made a huge impact with us, so it's normal that he should receive calls from elsewhere."
The 22-year-old struck six goals in 16 league appearances, providing three assists, and helped the club to the semi-final of the Coppa Italia.