Villa had fought since the start of the year to register the 6ft 4in Madjo following his £10m arrival from Metz.
Under-18 players cannot complete international transfers, but the club argued that since Madjo was born in London he should be exempt – despite having three caps for Luxembourg in friendlies.
Since those games, he accepted an England Under-17 call up last year and has four goals in nine appearances, scoring on his debut against Venezuela.
With England searching for their next No9 once Harry Kane eventually retires, Madjo could force himself into the conversation in the coming years.
Fifa does allow some exemptions for minor players to make moves, including if a 16 or 17-year-old transfers within the European Union – something the UK is no longer part of.
The battle was finally won just eight days ago – after Villa took their case to CAS in July – and Madjo was added to the travelling party for Salzburg, having not been included in the initial squad.
With Ollie Watkins rested after World Cup duty and Tammy Abraham left on the bench, the youngster was given the responsibility up front by Emery having scored four pre-season goals.
His strike at the Red Bull Arena, holding off Ecuador international Willian Pacho to volley in John McGinn’s cross, was arguably the hardest chance of the trio he had in a first half full of promise.
It cancelled out Khvicha Kvaratskhelia’s opener for PSG, only for Desire Doue to hit a second-half winner after a video assistant referee (VAR) check.
Before his goal, the hulking Madjo had earlier planted a free header over from three yards and shot wide, but was a constant nuisance for 11-time Ligue 1 winner and Brazilian centre-back and captain Marquinhos.
“He played because he was showing us his capacity to work and be demanding,” said boss Emery. “It is not just about scoring goals, it’s about defending and how he has the commitment and discipline tactically. He was doing that.
“I wanted to play with young players and watch their progression. Brian started scoring goals [in pre-season] against Walsall and Real Sociedad.
“Now the reality is we have shown their capacity, but they have to keep being consistent, demanding and keep performing at the level they were.”
Villa have been looking to sign a new striker – Chelsea’s Nicolas Jackson has been a summer target – but Madjo’s availability now gives Emery a different option and dilemma before the Premier League season opener at Brighton on 23 August.
“You’ve got a young player coming, especially on a stage like this one, against PSG, getting in those scoring positions and not allowing that disappointment to stay with you,” former Villa striker Dion Dublin told BBC Radio 5 Live.
“I saw Brian Madjo, he had his hands on his head for a couple of seconds and then he got into another scoring position.
“He was brave enough to get back into that scoring position and if he keeps missing it’s not frightening him. He’s happy to have that pressure on his shoulders. I’ve been there, you need to be able to deal with it. At 17, he seems to be doing it easily.”
Madjo – and fellow teenage forward George Hemmings – were given their chance against PSG with Emery insisting he would not rush Watkins, Emi Martinez and Ezri Konsa back for the game.
The trio are due to return next week after being given a month off following the World Cup, with Martinez losing the final with Argentina and Watkins and Konsa helping England to third place.
Madjo expertly deputised for Watkins, despite missing his earlier first-half chances, until he was replaced by Abraham with 18 minutes left.
Whether Konsa will still be a Villa player for the league opener against Brighton remains open to debate with Arsenal expected to step up their pursuit of the defender.
Villa have already sold Morgan Rogers in a record £117m deal to Chelsea and Youri Tielemans to Manchester United, and Konsa’s departure would be a another bitter blow after winning the Europa League last season and qualifying for the Champions League.
Keeper Martinez will stay, despite interest from Juventus this summer.
Emery added: “We decided to play this match with the players who were ready to play, were fit and trying to avoid some risk with some players. It is very important how we are going to manage the players.
“I am trying to use my experience to avoid some mistakes which i can understand can happen.
“I can feel free with my decision and proud of the players.”