In the good old days of leasing and inflated RV’s (pre-2020) we did see a few cases where BMWFS would offer a lower buyout than the RV. IIRC this was not something you could negotiate, but rather an offer that would appear in your account at BMWFS online usually in the last 3-6 mos. of your lease. Take it or leave it.
For obvious reasons I’m not aware of this happening at all since then.
So to answer your question, yes, at the current time your buyout price (RV) is non negotiable.