She says I owe her the full amount?

Please Read the end too. I was invited to hen night, the girl organising it said I had to pay her £30 right now as she was booking it now, I told her I couldn't pay her right now so she agreed to pay it for me and if pay her back once I was paid in a few days. Turns out, I couldn't go so I told her and she wanted my £30. I asked to see an invoice of what she had paid but she refused saying it didn't matter if she hadn't paid it in full? Eventually she sent me the invoice and she had only paid a deposit of £75 (22 people were going and we're giving her £30 each so I divided £75 over 22 and said that what I owe her (as she hadn't actually paid in full) she says I'm still due her the £30 but I'm saying I'm not since she's only paid a deposit and I will give her £3.40 which is what I'm due her from the deposit. Please someone tell me this is right as trying to explain to this girl is driving me mad. She's already admitted she's skint as doesn't have a job right now.