Can I cancel a race with a charity?

I have just seen a post in this forum similar to my situation: about a company supporting an employee to run a race. I received a registration number from the charity which asked me to pay for my entry (£45). I registered online on the organiser website and now I am in. But the problem is I cannot raise the money for the charity. I have never registered with the charity in the first place. So they don’t know me. I understand that the charity paid for me already. Who should I pay for my race if I cannot raise the money: the organiser or the charity? What if I don’t run: should I have to pay the charity back? It looks like I can run for free but I don’t want to take advantage of the charity. Thanks for your help. Stacey