What happens when you talk to Lt Robin in Ars Magicana? You need to get Tingalf arrested before you can report back to Trevor.
Just having a treasure map in your inventory should be enough for a dialog option to show up when talking to Mpenzak. I think you can ask him about his travels, or something, then ask if he has seen anything like the map you have. Go through all available speech options and something should come up about it, anyway.
Burning down his cart will not effect your ability to get the map piece.
Silver Armour and Treasure Maps
