App store annoying issue --'

Hey guys, I own a mba late 2011 & my brother a mbp retina 2012, he bought the GTA san andreas app from the app store, copy'd it on to a usb key & gave it to me... (never worked btw) but now as i charged cash on my apple store account to buy it, (i cleared the copy'd app from my app folder and HDD).

now pointing at the app in the MAS it doesnt offer me to buy the game but to update, wich requires my brothers Apple Account...

The question is : How do i clear the cache folder wich tells the MAS i have GTA SA on my computer... so i will be able te buy it legally, or will i have to wipe & reinstall ... again ...

(I use a 3G network to connect to internet so i really hope not)

Cheers