Even by the standards of soccer’s murky multibillion-dollar player trading market, where values are at times little more than crowdsourced guesses, one deal in the summer of 2020 stood out.

It was not necessarily the tens of millions of dollars that the Italian league giant Napoli was paying the French club Lille to sign Victor Osimhen that attracted attention. Few would have argued that Osimhen, a young Nigerian striker with a vast upside, was not worth a premium price.

Instead, what raised eyebrows was the valuation placed on four little known players who would be making the opposite journey to northern France: an aging second-string goalkeeper from Greece and three young players of little pedigree.

Official documents show Lille banked 71.25 million euros (just over $81 million) for Osimhen, a rising star now flourishing in Italy’s top league. As for the others, a group that sold for — on paper at least — 20.1 million euros ($23 million), there is now barely a trace. Only one, goalkeeper Orestis Karnezis, 36, has appeared in a match for Lille. The other three are all back in Italy. Two currently play in the lowest rungs of Italian soccer. One now plays semiprofessionally.

A year later, no one seems able to clarify how the numbers ever added up. And that is a problem because Italian financial and soccer authorities are now asking the questions about dealings that may have broken financial rules, if not criminal law.

The investigations in Italy, amid whispers of market abuse by clubs — teams that are in some cases publicly traded companies — are only the latest attempts to try to shine light into the dark corners of soccer’s player trading system, a $7 billion industry that has often defied regulation. What they are finding is a hall of mirrors in which lax regulation, creative accounting and even ghost clubs can distort the market, and where even insiders are often unable to answer a simple question:

What is a player worth?

The Italian soccer authorities have so far flagged 62 player trades across two seasons, including the one involving Napoli’s acquisition of Osimhen. But it is Juventus, by most metrics Italy’s biggest team, that faces the most scrutiny: Investigators are looking at 42 trades by the club, largely connected to movements of little known athletes from its reserves or youth teams, but also including some high-profile trades with two of Europe’s biggest teams, Barcelona and Manchester City, that have faced their own challenges meeting financial control rules.

Juventus has not commented beyond telling shareholders in its most recent financial report that Consob, the Italian equivalent of the U.S. Securities and Exchange Commission, has “initiated an inspection: of its income related to player trades.

Napoli’s owner, the film producer Aurelio De Laurentiis, said he was not concerned by the probe. “I’m not worried because I’m a warrior,” he said by phone.

Among the deals under scrutiny in the Italian investigation, according to documents reviewed by The New York Times, is a swap between Juventus and City of the defenders Danilo and João Cancelo, as well as another that allowed both teams to register gains of 10 million euros for exchanging two teenage strikers before either had made a first-team appearance.

Juventus made a similar agreement with Barcelona in which two little known South American forwards switched clubs, and the authorities are also looking into a 2020 deal involving midfielders Arthur Melo and Miralem Pjanic that the clubs claimed was worth more than 130 million euros, or about $148 million. Even at the time, the deal’s main value seemed to be as an accounting exercise, rather than a sporting one.

In Napoli’s case, the trade under scrutiny allowed the club to reduce some of the costs — on paper at least — associated with adding Osimhen, one of the most expensive arrivals in the club’s history. Like those of its domestic rivals, Napoli’s balance sheet must meet certain financial requirements set by the Italian federation and cost-control regulations related to the Champions League, European soccer’s richest club competition.

By sending the four minor players to Lille as makeweights in the Osimhen deal, Napoli was able to show the 20.1 million euros as an intake of millions on its books, a practice known as plusvalenzaor capital gain, that has in recent seasons become commonplace in transfer dealings. The practice has become even more valuable during the coronavirus pandemic, when club revenues have cratered because of closed stadiums, canceled matches and economic unease. Those same forces have affected the transfer market, cutting prices and restricting movement — making it even harder for big clubs to balance their books to keep within the rules.

Paolo Boccardelli, the head of Italian soccer federation’s supervisory body, known as Covisoc, wrote to its prosecutor last month advising him to start an investigation of transfer market dealings in Italy. In the letter, reviewed by The Times, Boccardelli raised an obvious point, and one that — privately at least — officials for the clubs involved in the probe have been eager to point out: No one can really say what a single soccer player is worth.

For years, billions of dollars were shifted around the world by soccer clubs buying and selling talent, with the market price determined by subjective calculations made by those involved.

“Identifying the fair value for the performance of a professional athlete is notoriously a difficult task and not supported by an adequate level of scientific evidence,” Boccardelli wrote in his letter. In essence, he was saying what others have said: The values that go onto club balance sheets are often fake.

“We can say in a lot of cases it is a sort of fiction,” said Pippo Russo, the author of a book about the transfer market who investigated and highlighted several curious trades involving Italian teams, including some of those now being flagged by the regulators. “They need to do this because they need to have annual accounts in order.”

Russo said that while “Italy has the copyright” on the plusvalenza system, he has seen the practice exported across the European soccer landscape, highlighting deals involving teams in Spain, France and recently Portugal, where two top division teams swapped two young players with hardly any experience, but only after valuing them at millions of euros apiece.

The involvement of Consob, the Italian financial regulator, is notable then, since its interest in the dealings of Juventus, a listed company, could carry serious consequences if the club is found to have broken the law. The soccer prosecutor’s powers are limited to sporting penalties, like points deductions and in some cases even demotion.

In 2018, for example, an investigation discovered Chievo Verona had for years engaged in a scheme in which it inflated the value of youth players traded with another club, Cesena. The deals, worth millions of euros in total, allowed each team to meet registration requirements to obtain licenses to play in the professional leagues.

Yet while Chievo was punished, some team owners in Italy have expressed frustration that rules are often changed or not enforced to protect the most successful clubs. Last season’s champion, Inter Milan, for example, secured its first title in 11 years even though it was unable to meet its payroll. The league, citing the coronavirus pandemic, had relaxed its regulation on wage payments halfway through the season.

Gaia Pianigiani contributed reporting from Rome.