I think that trick must have been fixed. The English version of the game came out after the German version, and I don't recall anyone reporting this in the English forums. It is certainly not possible to do in the English version of Divinity 2 Developer's Cut.
In English the belt that completes the quest is called 'Power Belt' and can be found on a bench in the far room in Louis' storage area. The other belt you mentioned sounds like it is just random loot.
Willkommen im Forum.
