Why would they need a source code? That's a complete nonsense. It's like you buying a car and then demanding that they give you their blueprint and source code for the car.
After snooping around on the web, I found this. It seems you can license the algo via QuantConnect's Alpha Streams. Not really sure on the specifics, so don't skimp on your due diligence before licensing. How to Submit Your Alpha to Alpha Streams
Are you able to compile it to a .dll or build it into a stand-alone .com application for them to test it? I mean if they just want to test it to see how it works, they don't need the source code. Once they buy it, they can get all of the source code they want. Right now you just need to give them a synopsis of what the algo does and its features and etc. and that should be sufficient for them. Asking you to give them the secret sauce before they fork over the money in the name of "wanting to test it" sounds fishy to me imo.
The binary code can be decoded. If the algo is really great worth 1 billion, it will be reverse engineered. Send them signals instead.
Are you sure they want your algo or they just want you to work for them? Your skills/knowledge maybe more valuable to them than your algo.
Ignore all the previous advice. If you need a connection at Two Sigma that works with prop traders (and you don't reveal your source code), DM me. Regards, PTR