If cash I would respond. Your receipt is sitting here, when you bring payment you can collect it. I would also consider something along the lines of "when you disappeared without payment I no longer knew how to get your receipt to you so was waiting for you to be in touch about your owed fees so I could get it to you".
I would back date the receipt to when she dissapeared then i would hand write a note on the receipt. A note to you/CRA that you lost contact with her and did not know how to get the receipt to her. 'Unable to locate-lost all communication abruptly".
I doubt it would hold in court but it makes it appear it was their dissapearing that resulting in a late receipt. Not you purposely with holding it.
That said, you DO need you get it to them whether paid or not. BUT I don't know there is much they can do if you kind of attempt to hold it until paid.
If you do NOT have a signed contract you are likely best to just seek the money for days care was provided. Some money is better than no money.
I wouldn't do this. It's our responsibility to mail them out if someone isn't in our care any more to their last known address so having it sitting in your possession will show you didn't do that.