Uggh, that would be tough. I did just find this article. If true, it sounds like they actually can't collect dollars until they sign the LOI. Who knows, it's the wild west :)
https://www.insidehighered.com/news/quick-takes/2023/07/18/missouri-law-will-open-nil-profits-high-schoolers#:~:text=The%20law%20will%20allow%20athletes,it%20will%20take%20effect%20Aug.
"The law will allow athletes to begin profiting from endorsement deals while still in high school—as long as they sign a letter of intent to attend a public university in Missouri. Passed by the state Legislature in May and signed by Governor Mike Parsons yesterday, it will take effect Aug. 28, as the fall semester—and football season—gets underway"