2003 TT Sport Pkg 18" vs. 17" wheels question

I am looking at buying a like new 2003 TT Quattro 1.8T 225HP 6 speed with the sport package. The car is perfect except who ever had it before didn't know where the curbs are so the 18" stock Audi wheels are really beat up. The dealer also has the same year car without the sport package so the wheels are 17" and I asked him to trade wheels for me on the vehicle and he said he couldn't do that because by putting the 17" wheels on the car with the sport package would mess up the Quattro and computer system and the car wouldn't run right? Is this true? Doesn't seem right to me I thought as long as you had the same size wheels on the front and back there should be no issue. Please advise as this 2003 only has about 19,500 miles and is in very good shape except these 18" wheels.... NADA has clean retail at $24,225 and Trade-in at $20,975 and they are asking $21,950 (seems like a pretty good deal to me but the dinged up wheels are bothering me.) Please advise.