The fun thing is, everyone here is calling this a scam, and while its clearly not what the investors intended, being able to park crypto funds in a way that is indexed to a specific USD amount is a legitimate service that has value: see the idea (but not the execution) behind Tether.

If he had just come out and said 'give me Ethereum and I'll give it back in 6 months at the same USD exchange rate' there are a non-zero number of people who would have taken him up on that, maybe even paid for the service.