Customer overpayment

Customer overpayment

Hi,
I have read through some of the threads on overpayment but I'm still not sure what I should do in my case.
I have a regular client who has accidentally paid his last invoice twice.
He is happy for me to retain the additional funds as a credit to him. 
So I need to;
a) account for it (the original invoice is already declared fully paid.)
b) be able to include it in future invoices. It will probably take 2 or 3 more work activities for me to  be able to bill him for the full amount.

Thanks

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