Q: Can I use the existing balance of my iTunes account to subscribe to iTunes match?

I bought a 50 dollars card, and after include this into my apple ID aacout, i went to suscribe to itunes match.  I don´t want a credit card associated, and I noticed that there is an option to trade itunes cards to suscribe.  It is too late for may card because I already trade it, but I bought it just to suscribe!  How can I use that money now to get Itunes Match????