The Seattle Seahawks placed Lynch on the retired/reserved list last May. If he decides to come out of retirement, they would retain his rights under the terms of his last contract, which has two years remaining on it.
Lynch would be due a base salary and cap hit of $9 million in 2017. He would be in line for $7 million in base salary and a $3 million roster bonus in 2018.
So what would the Seahawks' options be if Lynch decides he wants to play again? One option would be to hold on to Lynch. This is the least likely possibility, since they signed RB Eddie Lacy this week and would have to pay Lynch $9 million.
Their second option would be to trade him. This one also seems unlikely, because the Seahawks would have very little leverage. They have about $14.7 million in cap space, and Lynch would take up $9 million of that. The Seahawks are still looking to add free agents, and they need money to allocate to draft picks (around $5 million).
In order to execute a trade, they would have to convince the Raiders to take on Lynch's contract and agree on the terms of compensation. The Seahawks could hold out and try to force Oakland's hand, but the Raiders would have to desperately want Lynch in this scenario.
The final option (and the most likely) would be for the Seahawks to release Lynch. They signed Lacy this week and also have RBs Thomas Rawls and C.J. Prosise on the roster. Lynch turns 31 in April. Seattle may initially try to explore a trade, but assuming the Raiders don't budge, the Seahawks would likely be forced to release Lynch and move on.