The $30 will dissapear when you withdraw. It's tempoary credit you can use to bet with, like a never-ending freeplay (until you lose). Your max bet atm will be $1830, without the $30 it would be $1800. If you bet $1830 at +100, you'd get $3660 back. $30 of that would be on credit, so you could withdraw $3630 ($30 more than if you didn't have the credit to bet with).