If my parents did not withdraw their CPF and they meet all requirements to withdraw anytime, can they withdraw and then I do voluntary top up to get tax relief?

Yes, you can do that. But a few things to take note of:

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1. The tax relief for top-up to parents' CPF is limited to 7k. So if you contribute 10k, you only get a 7k reduction of taxable income.

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2. Assuming your parents are above 55 years old (since you said they can withdraw anytime), you will only get tax relief if their RA balance is below FRS. If their RA balance is higher than FRS, you don't get tax relief.

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(source: https://www.iras.gov.sg/IRASHome/Individuals/Lo...)

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3. When you topup cash to your parents' RA, the amount topped up can thereafter only be withdrawn slowly via monthly payouts, it cannot be withdrawn in one-shot immediately after you top-up.

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Quote from CPF webisite: "Top-up monies, CPF LIFE Bonus and Deferment Bonus are specifically meant to increase members’ monthly payouts under the Retirement Sum Scheme/CPF LIFE. Hence, they should only be streamed out in the form of monthly payouts."

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Graphical illustration from CPF: https://www.cpf.gov.sg/Assets/members/Documents...

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Numerical analysis done by Heartland Boy: https://heartlandboy.com/money-withdraw-from-yo...