Why The Cash Is Less Than You Expect
When an HDB flat is sold, the sale price is paid out in a set order. The outstanding housing loan is repaid first. Next, the CPF savings each owner used for the flat go back to that owner's CPF account, together with accrued interest: the interest that money would have earned had it stayed in the Ordinary Account. What is left, after selling costs, is your cash.
The CPF refund is your own money returning to your own account, so it is not lost. But it is not cash in hand, and after many years the accrued interest can be close to the principal itself.
If the flat is sold at market value and the price cannot cover both the loan and the full CPF refund, CPF Board says you refund only what remains after the loan is repaid. You do not have to top up the shortfall in cash. You would still need cash for the agent's commission and legal fees.