Since the turn of the year, Villa have been trying to register the 6ft 4in Madjo after his £10m switch from Metz.
Players under 18 are not permitted to complete international transfers, but the club maintained that his London birthplace should qualify him for an exemption, despite his three friendly caps for Luxembourg.
After those appearances, he accepted an England Under-17 call-up last year and has scored four times in nine outings, netting on his debut against Venezuela.
With England eventually needing a successor to Harry Kane, Madjo could put himself in contention in the coming years.
Fifa provides limited exceptions for minors, including transfers within the European Union for 16 and 17-year-olds — a route unavailable to UK clubs since Brexit.
The fight was finally won eight days ago — after Villa took their case to CAS in July — and Madjo was added to the travelling party for Salzburg, having missed the initial squad.
With Ollie Watkins rested following World Cup duty and Tammy Abraham among the substitutes, Emery trusted the teenager to lead the line after a four-goal pre-season.
At the Red Bull Arena, he held off Ecuador international Willian Pacho and volleyed in John McGinn’s cross — arguably the toughest of three promising first-half chances he had.
That strike levelled Khvicha Kvaratskhelia’s opener for PSG, before Desire Doue grabbed a second-half winner following a video assistant referee (VAR) review.
Before scoring, the powerful forward had headed a free chance over from three yards and shot wide, yet he continually unsettled Brazilian captain and 11-time Ligue 1 winner Marquinhos.
Boss Emery said he selected Madjo because he demonstrated work rate and tactical discipline, stressing it was about more than goals — defending, commitment and structure also mattered.
He added that he wanted to give young players opportunities and highlighted Madjo’s pre-season goals against Walsall and Real Sociedad, while warning they must remain consistent, demanding of themselves, and perform at the same level.
Villa have been pursuing a new striker — Chelsea’s Nicolas Jackson has been a summer target — but Madjo’s eligibility now gives Emery another option and selection dilemma before the Premier League opener at Brighton on 23 August.
Former Villa striker Dion Dublin told BBC Radio 5 Live that, on a stage like this against PSG, the youngster kept getting into scoring positions and didn’t let early misses linger.
He noted Madjo briefly showed frustration before immediately seeking the next chance, displaying bravery and a willingness to shoulder pressure — something he appears to handle with ease at just 17.