While Messi's feted strike partners Luis Suarez and Neymar have each signed new deals until 2021, the Argentine maestro's contract runs out on June 30 2018, giving Barca a window of 18 months to prevent him leaving the club for free.
Messi earns a reported 22 million euros (19.07 million pounds) per year after tax from Barca, while Neymar and Suarez's salaries are believed about 25 million euros after their latest contract renewals, and Grau said any new deal for the Argentine would put considerable pressure on the club's finances.
"Barcelona has to analyse this situation with a cold head and common sense. Barca can't exceed 70 percent of its budget on wages and therefore we have to make the numbers add up," Grau said on Wednesday at the Forum Europa.
La Liga agrees budgetary limits with each club at the start of every season and its regulations, known as economic controls, prevent club's from spending over 70 percent of their budget on wages.
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Barca's budget for this season is a record 695 million euros ($730.7 million). The Catalans pay the highest wages in Spain and the second highest in world soccer behind Manchester United, according to the latest publication of the Global Sports Salary Survey (GSSS), which said Barcelona players earned an average annual wage of $7,456,800.
Grau said one way for Barca to renew Mess's contract was to increase revenues. Their latest sponsorship deal with Japanese retailer Rakuten, the biggest in world soccer, will earn the club 55 million euros per year.
The club could also sell other players to in order to create a new contract for Messi, who is the top scorer in La Liga this season with 13 goals.
"One option is to increase our revenues, as our economic strategy forecasts. We want to have the best players around, but perhaps we have to prioritise," added Grau.
"The club wants the best player in the world to stay at Barca. I would like to ease the concerns of club members and supporters but we have to use common sense."
(Reporting by Richard Martin, editing by Pritha Sarkar)
