There are several special provisions in the CBA that affect contracts which extend beyond 2020. The two main rules are the Deion Rule and the 30% Rule. Jason Fitzgerald of Overthecap.com has written about the 30% Rule in his particularly fine article, "Carson Wentz' New Contract and the CBA," which points out some loopholes teams can use to circumvent the rules You can read Mr. Fitzgerald's article here.
The Deion Rule:
In 1995 Deion Sanders signed a 7-year contract worth, on paper, $35 million. The contract scheduled $1M in base salaries over the first three years to go with a $12 million signing bonus.