What should I do with $45k in OA for HDB downpayment? Should I keep $10-20k (withhold) or invest that amount in CPFIS?

If you are confident in being able to consistently achieve a yield above 3.5% every year, do go ahead with investing the funds. Perhaps consider investing into s-reits with strong sponsors and consistent dpu. I was invested in Lee Metal for a period of time before their delisting and they are very consistent with their 5% dividend distribution every year. I've since switched to reits offered by Mapletree and Frasers.

Withholding 20k will give you 3.5% interest p.a.

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However, do calculate your monthly mortgage repayment. 20k @ 2.6% for 30 years is 80/mth less to pay and more relevant if you're paying with cash.

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Investing would mean you'll need to constantly beat 3.5% net of fees to be worth it.

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In the long run, I'm quite confident it can. But it's up to your risk appetite.

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So cashflow first. Then confidence in performance returns second.