Help needed with bank rec for novice

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Can someone help? I was catching up with long overdue bank recs. I found an invoice that looked like it hadn't been paid but I had accidently marked it paid on the wrong date. 

Anyhow my attempts to correct it made it worse. I now have a Customer Receipt with no invoice that has €400 left to allocate and I cant get my bank rec to balance. I'm also getting the message "You can’t edit this transaction as it has an associated payment or refund.". Is there any way I can make this go away?