This thread might give you some insights into the issue: Is this normal? Our dog in a "wohnquartier".
Essentially, the complainer has legit grounds if you allowed the dog to urinate on other people's property, or on common property in a residential area.
Dog owners (and those who have temporarily taken on that responsibility) have to get a bit creative if one does not have a private garden.
I would assume the dog's owner has already figured out where the 'non-objectionable' areas in the neighborhood are - get in touch with the dog's owner and ask where he normally takes his dog for the daily constitutional. If you can't get in touch with the owner, just look around for other dog walkers in the neighborhood and ask.