Contract renegotiation has been forbidden under the NHL CBA since adoption of the hard cap in 2005. That is one of the core tenets of the unyielding system.

Except, apparently, when it comes to renegotiating the CBA itself. Because just four months after agreeing to a six-year extension in early July, the NHL is requesting the NHLPA revisit critical areas in the CBA as a prelude to opening the 2020-21 season, The Post has learned.

The union held a virtual executive board meeting Wednesday afternoon during which team player reps were informed by union leadership that the league is asking the players to defer an additional 13 to 16 percent of base compensation for the season on top of the 10-percent deferral that was originally negotiated in July.

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The request delivered from Gary Bettman to Don Fehr was accompanied by league “liquidity” concerns, according to multiple sources that included one person on the conference call. But the request for additional deferral is not the only concession the NHL is seeking from the players.

Indeed, the parties originally agreed to cap escrow at 20 percent for this upcoming season; between 14 and 18 percent (based on HRR) for 2021-22; at 10 percent for 2022-23; and at 6 percent for each of the final three seasons of the deal.

But now, the NHL is asking that the union agree to increase the escrow cap to perhaps as much as 9.5 percent for those final three years of the CBA.

Understand: Four months after negotiating a six-year deal in the midst of this ongoing pandemic that took into account potential economic distress created by the pandemic, the NHL is claiming it cannot live by the deal. It is impossible to cite a precedent in pro sport labor law history. The NHL is asking for a do-over.

We are told that the requests to renegotiate deferrals and the escrow cap annoyed, if not angered, the union membership participating on the call. At the same time, the NHL requests were not delivered in the form of an ultimatum. The union is likely to identify issues, perhaps systemic issues, it would like reopened in a renegotiation. The original 10-percent deferral is due back to the players without interest in three equal, annual installments beginning in October 2022. Perhaps the PA could negotiate for the full 23-percent deferral to accrue interest.

The Post has learned that a preliminary 60-game schedule was delivered to the league and to the PA for review within the last 48 hours. Sources report that there has been ongoing dialogue and progress between the league and the union regarding the 2020-21 format, schedule and health protocols, and that neither side believes the NHL’s Jan. 1 targeted puck drop is yet threatened by the CBA-related talks.

We’ll see.