Sporting director Ole Book moved for the 19-year-old on deadline day, agreeing a season-long loan without an option to buy for around €5 million, and the deal delivered an instant return when Nwaneri played a key role in Dortmund's 3-2 comeback win over TSG Hoffenheim on his debut.
The England Under-21 international, introduced after Filippo Mané's injury, supplied the cross that led to the winning goal.
The mood was dampened a day later, however, when it emerged that Arsenal could recall the attacking midfielder in January via a clause written into the loan agreement.
Speaking on the Ruhr Nachrichten's Dortmund podcast, insider Kevin Pinnow played down the scale of the threat, revealing two previously unreported details of the contract.
According to Pinnow, Arsenal are contractually barred from activating the recall clause on short notice during the January window itself; instead, the Gunners must announce their intention to trigger it well in advance, "before the start of the transfer window," though he declined to specify an exact deadline.
The second detail is financial. Should Arsenal exercise the option, Dortmund would be compensated: the loan fee already paid would be effectively wiped out, reducing it to zero, and the Premier League champions would additionally be required to cover the remainder of Nwaneri's wages for the season. That arrangement offers Borussia Dortmund some financial protection even in the event of a January recall, softening what would otherwise be a blow to a move that has started so promisingly.
Nwaneri, a Hale End academy graduate who became Arsenal's youngest-ever player when he debuted at 15 years and 181 days old in 2022, spent the second half of last season on loan at Marseille, where he scored twice in nine Ligue 1 appearances, before returning to south London over the summer.
Arsenal had been reluctant to sanction the move given the strength of competition Nwaneri already faces in attacking areas at the Emirates, with manager Mikel Arteta acknowledging last month that game time was the priority even if it meant a temporary departure.
"I want him to be at the club, but something is in the medium and long-term, what is the best for the player as well," Arteta said.
"Ethan needs to play football. If he stays here, it's because we can guarantee that he can have those minutes. Otherwise, it's just something that is not good for anybody, I think."
Dortmund's interest in the teenager was long-standing, having reportedly hoped to sign him as far back as last summer before he opted for the Marseille loan instead.