It's impossible to calculate exactly how much interest I would save. This is because one can't predict exactly how long the construction would take. (The developer is paid in instalments by the bank, as the developer completes each phase of construction. The instalments represent my interest-free loan - interest free while the property is still under construction, that is).
But I did a rough financial model .... The number I got was $23,100.

Quite close to $20,000!
I want to add that this kind of "interest absorption scheme" is not at all common in my country. I googled around and found that this kind of scheme is just making a comeback just this year (starting around March / April),. Here's an
article I found, about it.
An amazing result for my Day 4 manifestation!!
Now what I need to decide is whether I will really buy that property or not.