The Baggies are keen to sign the 23-year-old on a season-long loan with an option to buy at the end of the season.
But Sporting want the Baggies to agree to an obligation to buy Pereira next summer, regardless of whether Albion are promoted or not, for a fee in the region of £9m.
Pereira would need to feature in just over half of Albion’s league games in the season to activate the purchase.
Albion’s parachute payments fall to £15.5m next year, making a £9m outlay on one player next summer a gamble it’s understood the Baggies are not prepared to take.
New boss Slaven Bilic is known to be a fan of Pereira, and talks with Sporting continue with the aim of completing a deal.
Albion still want to sign him this summer but the Portuguese club will need to agree to a change in their terms.
Pereira is a versatile forward capable of playing on either wing. After moving to Portugal from Brazil at an early age he came through Sporting’s academy.
He spent last season loan at FC Nurnburg in Germany, scoring three goals and laying on two assists in 16 starts.
He was nominated for the Bundesliga Rookie of the Year award even though Nurnburg were relegated.
The Baggies currently have Matt Phillips and Kyle Edwards on the wing, as well as academy products Rayhaan Tulloch and Jonathan Leko.
Although Oliver Burke can play there, he has been featuring up front in pre-season.
Albion are keen to add more attackers to their squad, and are also interested in Brentford playmaker Romaine Sawyers.
The Baggies have reportedly had an opening bid of £2.2m for Sawyers rejected by the Bees, who want closer to £3m.
Sawyers came through the Albion academy before making his name at Walsall under current Aston Villa manager Dean Smith. He followed Smith to Brentford in the summer of 2016.