Looking at a few websites and we can read about "cash over cap" which teams are using more and more to avoid the hard cap the NFL has.
Players bonuses are prorated so that Dak's 160MIL 4-yr deal includes a 66MIL bonus...which is pro-rated over the 4 years and per spotrac Dak's cap hit for 2021 will only be 22MIL, with 95MIL of dead cap $$ being kicked down the road to 2022 and beyond.
Cowboys will have somewhere around $25MIL in cap space left (14th in the NFL apparently) for FA signings and anybody else not under contract now.
Seattle is potentially in one of those situations. Should he be traded before June 1st, Seattle would take a $39m cap hit for this season (the balance of his pro-rated signing bonus) as opposed to $32m (19m base salary + 13m of pro-rated bonus) if he stays in Seattle.
The way this contract is structured, Dallas won't take a massive cap hit til 2023 & 2024.