OK good to see the dealer is making efforts to help. Let me comment and suggest more info for your decision.
1. Another member with the same wheels of the same sizes (you know him)
http://f10.5post.com/forums/showthread.php?t=1150651 says, he’s got the fronts ET25 (post #8).
2. Look at the pic. in the post #11
http://f10.5post.com/forums/attachme...1&d=1437184783 this is the proper shooting angle to see how the front wheel (ET25) are very much flush to the fender. Another 5 mm will make ET20 and a little poke.
3. See my front wheels. 20x8.5 and this is ET20. You can see Michelin PSS front tire has a triangle sidewall that pokes a little, while the rim is just level to the fender. Having 20x9 with the same offset ET20 will bring the rim level +6mm just to the tire’s edge.
4. Now see the rears 20x10 ET28 that is equal to 20x10.5 ET34. Here is pretty much clear you will be fine with ET35 or you will be tucked 10mm with ET45 (as discussed).
Let me make some conclusions:
- Dealer can be uncertain and can make mistakes. He cannot know every millimeter of every rim because he has a number of car brands with a number of models and a number of rim types. My own wheel dealer made mistakes regardless crazy price of the wheels. This is not cool, but this is life.
- The large threads here like this one:
http://f10.5post.com/forums/showthre...=664780&page=2 tell us the good offset for 20x9 front is ET25 that equals 20x8.5 ET20 (see my case). Also for rears 20x10.5 ET34-35 is fine.
- Your dealer tells you the ET25 doesn't exist but another member has it (so I trust), and it looks great.
- Measurements of offset is simple but people can skip it just looking at the specs. I did measure my wheels myself and sure about my own numbers.
I really want to help you, so I recommend to share this info with your dealer and work it out together. This is known issue: different wheel designs (especially shape of wheel barrel lips) can create a bit different ET and look differently, so after all calculations the final thing is to put the wheels on your car and see.
Talk to dealer, share your concerns and let him to commit for the test fit. If he insists on ET20 and it wouldn't fit, he will replace it with ET25 or ET35 whatever is really available.
Hope it helps
