Can you afford at least one at-home THC test kit from your local pharmacy? Who knows, maybe you're negative as of this moment. Why not scrounge the money, take the test, and find out. If you're negative, you can just stop all worrying right there and be on happily with your life.

If you're positive after the test, and you can't afford a synthetic, then why not substitute using a friend's clean urine? That's the best price of all....free. You'll need a hand-warmer and enough time to heat it up to the proper temperature. There's lots of advice on this forum about techniques for doing it successfully.