you will apply loan based on 550k (SPA price).

if you loan based on nett price (510k), there will be a difference between the purchase price and loan amount (this is what we call as differential sum) as maximum margin for a normal residential loan is 90% (90% of 510k = 459k, meaning u have to fork out additional 51k to pay to developer from own pocket).

if loan based on 550k and u pinjam 90% loan:-
purchase price - 550k
loan amount - 495k
pay to developer = 55k minus 40k (rebate on unit only) = RM15k (all add up to be 10%)

agent will usually tell you the nett price psf (after discount) so it sounds cheaper (and thats what u actually paid for the house).