Is your taxpayer the mother of this person?

"House sold in 2022 for 750K.  The 2-year use requirement not met."

Then it sat empty? If it was rented, who was reporting that activity?

"1/2 of the sale price"

You mean, the Payment. The funds, not the price.

"was received in 2022 and reported on 1099-S."

Is this half amount the total reported on the 1099-S? In other words, it was not sold for $750k? Or, the 1099-S split the sale to two parties, each being reported for half the sale proceeds?

"Per a legal agreement (made several years before the sale),"

What sort of agreement overrides the deed/title and a legal sale?

"1/2 of the sale price will be received in a future year upon the passing of the owner-mother of son."

It seems the mother sold the son's house, and kept half the proceeds?

"This could be an installment sale."

Between which parties? A buyer buys from the owner. Where does the mother fit into this?

"There could be a world of tax when the other 1/2 is received."

Or, you have a sale, then the son loaned money to the mother, and there is no house or property involved in that activity?

"I suppose I need the legal agreement before I make a final tax determination. But as of now, the future is the future."

No. Reality matters.