Barry left Goodison Park to join the Baggies in August 2017 and went on to enjoy two successful seasons with the club.
It was initially announced he moved to the West Midlands for an undisclosed fee.
But it has since been reported by The Athletic that Albion made no initial payment.
Instead, they agreed to pay a set sum for each appearance Barry made – with the total fee capped at £1.5million.
Despite making 55 appearances, that fee was never reached.
Barry was released by Albion at the end of last season after suffering a knee injury which required surgery.
But he has stayed with the club ever since to complete his rehabilitation.
Head coach Slaven Bilic then made clear he wanted re-sign Barry on a deal until the end of the season once he had proven his fitness.
Technically a free agent, Barry can sign for anyone else on a free transfer.
And Albion believe their payment plan with Everton expired at the end of Barry's initial two-year deal.
But Everton believe it still stands with the two clubs now locked in negotiations.
Bilic, though, is confident the issue will soon be resolved with Barry still set to be a Baggies player this season.
“Gareth has to sign a contract,” Bilic said when asked if Barry will feature for Albion’s under-23s on Monday.
“There is a small issue with Everton. It is all agreed between us and Gareth.
“But there is an issue with Everton, of course, a financial one.
“And until that is sorted he can’t sign a contract. And until he signs that contract he can’t “play for us, not even in the under-23s.
“It is very frustrating for both him and for us because he is training with us - all that he needs “is few games.
“It’s one thing to train but after a long break it’s another thing completely to play games on a big pitch.
“But that is the situation, that is the situation why he is not involved in any of the games.”
Bilic admitted both he and Barry are frustrated by the situation.
“It is disappointing and frustrating for him, it is disappointing and frustrating for me as a manager,” he continued.
“I spoke yesterday with Luke (technical director Luke Dowling) and Mark (chief executive Mark Jenkins) and Ke (board member Ke Xu) - we had a long chat about that and we are doing everything.
“But there is that little clause with Everton that’s been there since he came here, that we are trying to sort out and hopefully we will do it as soon as possible.”