Your terms are too generous. For example, I don't understand why the bolded wouldn't be a win for you.
I assume the operating theory behind this bet is that "Roy/UNC harms a recruit's draft stock, and highly-rated recruits would be advised to go elsewhere." (There is much-discussed anecdotal evidence of this in the way UNC tends to keep RSCI top-10 recruits around longer than other programs.) If so, then if Little declares but goes undrafted, that should be a win for you.
Furthermore, I would just state the bet like this: If Nassir Little is a first-round draft pick in the 2019 NBA draft, UrinalCake loses a pie. Otherwise, UrinalCake wins a pie.
No self-respecting UNC should turn down this bet considering Little is a consensus top-5 draft pick in early NBA mocks. I mean, either UNC fans believe the narrative or they don't. Which is it?