I don't recommend borrowing money. I would get the $20 worth with your money, then try to sell it to the friend for $30, if the friend backs out or doesn't get the money (things DO happen), then you will at least have the stuff paid for so that you can turn around and sell it to somebody else (or just throw yourself a party with it). You don't want to be the customer that's running around not paying for shit - that just starts to look bad. It's unprofessional. Let your friend look like the ass that doesn't have the money yet.

Better PR, IMO, and safer.