I can see how this mismatch of tenses is confusing. It's not impossible, though: you could say that if you had actually managed to return his pen to him at lunch time.
In the scenario where the person already had his pen back, the possibility of returning the pen this evening at the cinema would, in fact, be a past conditional. This is because it's a hypothetical unreal situation based on something that happened in the past. So, during the afternoon, it would be correct to say " [If hadn't returned the pen at lunchtime..]..... I could've returned it this evening".
Confusing as it sounds, this actually does make sense.