The Norton-Cuffy deal effectively funds itself, with Hull recouping a large chunk of the fee by sending Cody Drameh, who started all three of the Tigers' matches so far this season, to Genoa for £3.4m as part of the same swap arrangement.
It continues a busy summer at the MKM Stadium, where the club have now made 13 signings since securing an immediate return to the Premier League after a nine-year absence, a stay that has begun with back-to-back wins.
Norton-Cuffy, 22, began his career in Arsenal's academy before loan spells at Lincoln City, Rotherham United, Coventry City and Millwall preceded a permanent move to Genoa two years ago, initially on an undisclosed fee. He was part of the England Under-21 side that retained their European Championship title in Slovakia last year, and had also attracted interest from Everton earlier in the window before Hull moved to secure his signature.
Ansah's arrival represents a significant financial commitment for a newly-promoted club. The 21-year-old, who turns 22 in November, has signed a five-year contract in a deal reported at €15m (£12.8m) guaranteed, rising to as much as €27m (£23m) with add-ons — a package that would make him Union Berlin's most lucrative sale, surpassing the £20.5m Nottingham Forest paid for Taiwo Awoniyi in 2022. Union had signed the German Under-21 international from Paderborn for around €4m only last summer.
Ansah made an immediate impact in Berlin, scoring twice on his full Bundesliga debut against VfB Stuttgart — the first player in Union's history to score a brace on a starting debut — and adding further goals in each of the next three matchdays.
He finished the season with five goals in 36 appearances across all competitions, form that also earned him recognition at international level with Germany's Under-21s.
"I'm very excited. It's a dream come true for me to play in the Premier League, and for such a big club — it's really an honour for me," Ansah said. "I have heard a lot about the club, the players and the team, and I'm excited to play with the boys and to get to know them.
"I'm definitely a player who always gives his best and leaves it all on the pitch."
Hull City's deadline-day activity was not limited to Tuesday's deals. The club were also finalising a loan move for 19-year-old AS Roma striker Robinio Vaz, understood to be worth around €2m with a non-mandatory buy option set at €30m, as Hull's recruitment team continued working the phones into the final hours of the window.