This is a common financial practice in some fields -- you might have heard of something called "securitization" in the news lately. :-)

That said, applying it to a startup seems rather weird. Normally the way securitization works when future revenue streams are predictable, which is far from true for most startups.

I wouldn't call this a scam necessarily, but it certainly raises my eyebrows enough that I'd recommend reading over the fine print very carefully to make sure that you know exactly what you're agreeing to.