What % discount off MSRP on Special Order

My wife has an expiring lease (June 2019) on a '17 MB C300 4Matic Coupe. She wants another one, but searching even nationally I cannot find a 2019 equipped “exactly” the way she wants it (the Parking Assistance Package is now apparently rarely ordered by dealers, but she has it on her current car and loves it). We are in the NY/NJ/CT market FYI.

I’m working with a dealer that will order the car exactly the way she wants it (it will come in as a 2020 model in early to mid August) and we can get her current lease extended a couple of months. There is no pricing yet on 2020 models, but I’ve checked with multiple sources and there won’t be a significant increase nor any changes on the 2020 model since there was a facelift in 2019.

The question here is: How much of a discount do I ask the dealer to take on an order? He is obviously willing to place the order but tells me that we have to agree on a percentage before he enters the order.

I also realize that I can’t negotiate lease terms yet because we have to wait for the car to come in to know what rates, programs, etc are available in August. I will deal with that when I get there.

Yes, I also know I don’t have leverage here to get a great deal. But as they say, happy wife, happy life.