Key quote:
While the Mariners are theoretically setting aside $5 million each of the next five years, they don't have to actually put it into escrow or produce it. All they have to do is come up with the money, plus 5.5 percent compounded annually, when it's due. In the meantime, they can invest the money in hopes of earning more than 5.5 percent or -- and this is probably what Ichiro had in mind -- invest it in payroll.
In fact, not only do they not have to escrow the money, it would kill Ichiro's tax situation if they did so. An escrow would be treated as constructively received by Ichiro when funded, unless it qualified as a Rabbi Trust under IRS guidelines. In its present form the contract is only an unsecured promise to pay in the future, not evidenced by a promissory note, and is therefore eligible for tax deferral (as mentioned in the article). Under newer rules, the contract also can't provide for acceleration of payments, and I would assume that a future change to an escrow arrangement would be such an acceleration.