As Steve Miller once said "go on take the money and run".

I'm assuming you think the money would help fund the business and is needed, or you wouldn't be asking. Maybe to help incorporate, host the project, etc. So why not take it?

You said in a comment that they're a professional investor. I'd find a decent attorney that will work on a deferred retainer and do a quick convertible (there's no reason to go equity here) and keep your fees to a minimum.