I don't see how Lenglet would agree to that.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.
If we use your numerical example he can just say no and sit out his contract which would earn him 30m in 2 years. From then on he can sign for another club, obviously not on same wages but let's say 5m per season. Over 5 years he'd then make 45m instead of the 30m spread over 5 years you suggested.