It's probably because the 'now' is actually a 'just now' i.e. a short while ago. This refers to the very recent past (as recent as a few moments ago) when the speaker presumably did return the favour. It's not uncommon to use the past perfect to refer to a period ending in the very recent past. For example: " Jack and Emma have had a little boy!" "Oh! That's lovely. I hadn't even realised they were expecting a baby". In this case, the 'hadn't realised' period ended only a second or two ago - but it's still in the past. You could use a past simple instead ( "I didn't even realise they were expecting a baby") but not a present perfect. Why not a present perfect? Because the period ( of not realising) is over.

The reason for the use of the PAST PERFECT in this context is that the speaker is admitting to having done something he had not previously done. In other words. : I have returned (just now ). Until now - I had not.