Best he could do to get paid market value and help the cap is do a really long-term deal that escalates in the end (to force a redo).
Patrick Mahomes wasn't underpaid when he signed his 10-year, $450,000,000 contract at an average of $45 million per year, but by signing such a long-term deal his first two years were $5.3 million and $7.4 million (and the next three were all under $40 million). They kept the signing bonus low as well, but got $141 million guaranteed. That was a good deal for everyone.
Parsons is set to make $25 million this year. If he did something similar to Mahomes where the first two years were $5 million to $7 million, they could actually afford Garrett's final two years ($19.7 million and $20.3 million) with the savings.
If Parsons takes the typical four-year deal, then it most likely will include an astronomical second-year base that Dallas will turn into bonus to create cap space, but there's less maneuverability for spreading out the money. That is the typical contract structure for Dallas, so I don't expect to see anything create. And, of course, I don't expect us to end up with Myles Garrett.