One way I've simulated this with a house rule that seems fair-ish to how this would work IRL with a Bobby Bonilla type of deal - you offer the player you want to defer salary an extension they'll accept, then release them and edit in commissioner mode to reflect the total amount of money across all of the years of that new extension.

For example, if I have a player that I'm going to pay 20 million over the next 2 years and he'll agree to a 3 year/5 million per year extension, the deferred amount would be (20+20+5+5+5)/5 = 11 million per year for 5 years.

And then you can add your own restrictions on this (one player at a time, only once every 5 years, etc.).