They are correct the RD and MF are set by MB. You cannot negotiate this.
Remember that with leasing you are basically paying the depreciation of the car plus the MF.
Lets just say in this case that RD is 60% of MSRP on a 36 month lease with 12K per year and you have an MSRP of $70K. (I am rounding and overly simplifying these numbers just to be easier to follow. There will be fees included in your deal that I wont discuss.)
In this example you will calculate the depreciation. (($70,000 x .4= $28,000) or ($70,000 x .6= $42,000 then $70,000 - $42,000= $28,000))
So you be basically be paying back $28,000 over 36 months plus the MF.
$28,000/36= $777 per month in depreciation
Your payment would be $777 plus MF plus taxes
To get a lower payment you have to negotiate a lower payment or put money into the deal as cash or trade.
Now lets say you were able to negotiate a discount of MSRP of $2000 and maybe MB has a $1000 lease incentive.
Your new payment calculation would follow this logic:
The residual value stays the same regardless out the price you negotiate so it will still be $42,000.
But now you will be subtracting the residual value from the negoitated price - $67,000 - $42,000= $25,000
Now you will pay that $25,000 over 36 months- $25,000/36= $695 plus MF plus taxes
So if you negotiate a better deal, put money into the deal or MB has better incentives your payment will be better. The converse is true if you total cost goes up, you do not put money into the deal or MB stops incentive.