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When talking about contributions to an IRA, the IRS talks about "taxable compensation " not earned income.  Taxable compensation includes:

Wages, salaries, tips, professional fees, bonuses, and other amounts you receive for providing personal services are compensation. The IRS treats as compensation any amount properly shown in box 1 (Wages, tips, other compensation) of Form W-2, Wage and Tax Statement, provided that amount is reduced by any amount properly shown in box 11 (Nonqualified plans). 

The screenshot below shows Table 1-1 Compensation for Purposes of an IRA from IRS Pub. 590-A - Contributions to Individual Retirement Arrangements (IRAs)  [p. 6]

It seems as if the incentive would be a bonus, and not retirement income.   Since your employer included the first payment on a W-2, they may also do that for the second payment, but you need to check with them.