If you are covered by a workplace retirement plan at any time during the year, you follow the IRA contribution eligibility rules for the entire year. There is no way to say that certain dollars were earned after your retirement and follow different rules. You are covered by one set of rules for the entire year.
If you can't make a deductible IRA contribution due to your other income from the covered job, you can still make a non-deductible contribution. This creates a taxable basis in your IRA that you must keep track of, but that portion of your money won't be taxed when you withdraw it in retirement.
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