Who the heck in this scenario is your taxpayer? I have BTDT with situations where I know everyone involved and the real facts are so complex that I have to simplify, and in the simplification leave out the essentials - and I think that's the confusion here.

Dad gifted real estate to Son in 1998. For 24 years, Son owned the property and rented it out? Let Mom and Dad continue to live in it? At some point, Dad died?

Son sold this same real estate to Mom? or an unrelated third party? in 2022, pursuant to a pre-existing contract that allowed Mom to retain half the sales price for the rest of her life, for a price lower than its 24-years-earlier FMV? Was that price current FMV?

IRonMaN's question is "Who's holding on to the share that Son will get when Mom dies," and I think the answer is "Mom, who is also Buyer."