Whether this is usual, I unfortunately cannot judge due to lack of experience. However, I find the idea disturbing because, in the end, it is a "Wash me, but don’t get me wet" or "I want to have my cake and eat it too" issue. The investor wants the money now, wants no capital tie-up and no risk, and still wants to earn again later. If it does not become building land, he wins now; if it becomes building land, he wins a second time later. In the opposite case, if the standard land value should fall, he probably will not provide any compensation. Maximum profit without any risk is, in my opinion, somewhat odd.
That would be like getting divorced but demanding from the ex-wife that you get the lottery winnings if she should win in ten years. Or selling your rusty bus at the current market value but simultaneously demanding to get the quintupled value later if the rust bucket suddenly becomes as valuable as a Samba bus. Or selling stocks because you want money now but demanding from someone that you still want to participate in the profit if the price rises after the sale.
I might perhaps understand a 50/50 arrangement in terms of risk sharing, but as it is, this is far too one-sided to the disadvantage of the buyer. If the investor wants to profit from an appreciation, he should bear the capital tie-up, the opportunity costs, and the risk himself and hold the property for the next 20 years.