With all respect, "willingly signed" is not entirely accurate. Chase was drafted by the Bengals and he was subject to the rookie wage scale. If there were no NFL draft (which may legally be a restraint of trade, but that's another discussion), and he could be recruited, as high school athletes can, then he almost certainly would have gotten a much bigger rookie contract.
And Chase is playing under his contract this year, even though he was promised an extension and is not getting it.
That's exactly the point: extensions are contracts based on past performance. They are recognition that a player has vastly exceeded expectations, and that, relative to his currrent contract, he is vastly underpaid. The very low cap on rookie contracts makes this situation especially likely.