Legal document that gives right to purchase?

[Dan Wells]

I am in TX, I want to lock in a commercial property with a private seller, he doesn't want to go the traditional route of selling outright just yet, due to the property having items of value that need to be sold first. He estimates it will take about 2 years to do so. He doesn't want to do a lease back either, or owner finance. 

He told me that he promises to sell it to me for our agreed price once he clears out his items. lease with option to buy only last so long in Texas so not an option

So what other options do I have that will hold up in a legal setting? Contract notarized? Opinions please.