If he was willing to pay then always take the money lol. I mean, don't screw him over, but if he was willing to get you 20/30 dollars then just take it, it's your work you deserve it

Did you state any kinda terms with him? If you didn't then i'm not really sure where this stands legally, but you did make the beat, which automatically makes it copyrighted to you by law. It's just that you havn't registered it with the government so you have to prove it another way.
But this is for a mixtape right? I'm sure if you keep on at the dude to put a tag in there he will. And chances are he not gonna make much (if any) money from the tape anyways, so there wont really be a 'big thing' made of it anyways. But just keep on at him about the tag and you should be okay
