Manchester United refused to agree a pay parity clause when Paul Pogba signed for the club from Juventus.
The France midfielder became the highest-paid player at Old Trafford following the £89.3m switch from Turin back in the summer of 2016, earning him a basic salary of £300,000-per-week.
It is understood Pogba's agent, Mino Raiola, wanted to keep his man at the top of the United money tree, but club executives were determined to protect themselves against wage inflation at the time.
Pogba has been left firmly in the financial shade by the arrival of Chile forward Alexis Sanchez from Arsenal, with wages of £505,000-per-week and an annual bonus of £7.5m also payable to him for each of the next four years.
There have already been rumblings that Raiola wants to renegotiate and is unhappy with the huge cash discrepancy between his client and Sanchez, who is also five years older than the Frenchman.
United are playing down the prospect of a showdown and point to the three years Pogba still has left on his contract, although Raiola is in no mood to be conciliatory towards the Red Devils, having seen his player Henrikh Mkhitaryan pushed out the door to Arsenal ahead of Zlatan Ibrahimovic's anticipated departure this summer.