No there are rules on what contracts will be accepted by the league for FFP.
Barca could not sign extensions now where barely pay players for 2 years then 'over pay' them in years 3,4 and 5.
It's not that kind of a contract. Barca did a normal extension with Umtiti, devided the money they owed him in the last year on the new contract, meaning he cost them 5 times less per year in the books.
The club can do the same with Lenglet. If the club ows him 30m in 2 years for example and everyone understands no club is willing to offer him this salary, Barca can extend the contract without paying him a new salary, just deviding these 30m to 5 years, meaning he'll earn 6m per season, not 15 anymore. He gets the same money just over a longer period, but has a chance to find a new club with his new 6m wages.
It's not about overpay them in the last years, but an usual, even payment over 5 years.