Do I need to pay Additional Buyer's Stamp (ABSD) upfront when buying a smaller private property if I'm over 55, currently own one private property, and plan to sell it within 6 months of the new purchase?

ABSD liability depends on the buyer's profile and number of residential properties owned at the point of purchase. As you currently own one private property, ABSD will be payable upfront when you purchase the second private property.
However, Single Singapore Citizens aged 55 years and above may be eligible for a refund under the ABSD Concession for Single Singapore Citizen (SC) Seniors Remission, provided all remission conditions are met.
One of the eligibility conditions for ABSD refund is that the value of the second residential property is less than the value of the first residential property sold.
Do ensure that all conditions are met before applying for a refund under this remission.