Too honest for my own good?

I've just posted off a cheque to one of the publishers I work for to pay back money they'd overpaid me. It was an invoice towards the end of last year that got lost in the system. When I chased it up, there was a bit of to-ing and fro-ing via email and somehow, it ended up being paid twice. Yes, I did consider just keeping quiet and hoping they wouldn't notice, but somehow it just didn't feel right.

The relationship between a freelancer and the publishers they work for is very much based on trust. We rarely have proper written contracts for the work we do, so we have to trust the in-house editor to send us the work they've promised and they have to trust us to do it. And apart from the odd delay here and there, I haven't been majorly let down by anyone I work with in nearly 12 years of freelancing. So I figured it's really not worth breaking that trust now.

The accounts dept did offer to put a credit note on their system and take the money off the next job I do for them. But somehow the prospect of "working for nothing" at some point along the line felt like it'd be a bit demotivating! Better to pay the money straight back and just pretend it never went into my account.