Ouch! That sounds like a big hit doesn't it.
Part of the problem for both the current Panda and the 500L is that they are now significantly cheaper to buy new than they were at launch. Contrast this with the 500, which has only ever seen price rises really since it was launched in 2007 (I think this has finally tapered off now, but not massively).
My 2012 Panda 1.2 Easy cost me £10,050 with the various options added. If I bought the same car now it would be nearly £1,500 cheaper with all of the extra discounts.
I kind of agree with the dealer's comment about the 500X's residuals, but a lot will depend on whether it sells well or not IMO - the jury is out for the time being. I'm not sure the current Panda and 500L have sold all that well to be honest, at least not as well as expected.