As far as i'm aware, they have to match a price reduction if it occurs while your items are on layby. All stores are obliged, Myers does it as does Kmart....
The catch is, you have to be the one to bring it up.... they won't drop it automatically, and they won't offer it if your collecting withing the sale time.
I did it t kmart the other day, last time they had 20% off TF and SW as i'd laybyed Powerglide and some SW 2 packs... didn't even need the receipt, just went in and they did it.
What they can't do is add an employee discount after the layby is started, or alter it (in the case of kmart and the 15% employee they had the first weekend.... had to cancel my m Falcon lego and redo to get the 15% staff, but the price reduction from laybying before the sale was fine)