Will it be better to top up CPF to pay for HDB loan or just pay by cash?

Firstly, the only way you can top-up CPF to pay for HDB loan is through the Voluntary contribution to 3 accounts OA, SA, MA which are not entitled to tax savings.

So, the only source of CPF is coming from your OA contributions from Employment or Government awards (SAF, SPF).

It depends on what your opportunity cost is.

By using CPF-OA to fund your housing loan, you are giving up a 2.5% interest paid by the CPF board yearly. If you were already doing investments yielding 3% annually, then it is a no-brainer to use arbitrage since Housing loan is a good debt. Take note that investment returns are not guaranteed

But if you just want a peace of mind and happy with 2.5% return on OA. Go ahead and use your cash to pay for the housing loan. It will also help build your CPF faster for retirement needs.

Any amount from CPF that is used for property purchase will have accrued interest.

Meaning the interest that would have been received if it was not used for the property has to be paid back to your account when the house is sold. Depending on the loan amount and tenure, the accrued interest can be quite an amount.