You realize that there's a difference between "I totally came up with this plan and I want to do it and I seem to be oddly relieved if I killed that Menge with my own hands" vs "Hey, 'daughter who I apparently want to forsake', I am now going to talk you into doing something that you don't want to?"
The part in bold is something Geralt's character does not know in advance, there is simply a choice between:
1. Free my friend Dandelion.
2. I'm looking for this treasure…
3. Fuck it. Had enough of this masquerade.
Not knowing about the exact outcome of each choice, in my opinion it makes the most sense to choose 1 over 2. In the given situation, anything could go wrong at any time, that should be obvious even without the knowledge that there will indeed be no second chance. It is not like Geralt is having a tea party with Menge, he could easily grow suspicious and call the guards, and also every additional line of the conversation means more torture. Therefore, it is best to choose what one finds the most important the first. You are even warned to focus on Dandelion and that the treasure is of secondary importance:
Geralt: What about the treasure?
Triss: If we learn anything, great. If not…
Geralt: Thought you need coin from Dijkstra to help the other mages.
Triss: True, I do… Lots of ways to get gold, though. It's just not the most important thing now.
Regarding the comparison to taking (or not) the reward from Emhyr, my point was that one can indeed make choices for reasons other than maximizing gameplay related benefits (money, XP, and whatever else). It was not about comparing the morality of the two situations, even if I would not feel particularly good in Geralt's place about prioritizing some treasure over the life of one of my best friends and by extension my daughter, nor about using Triss' help to find Ciri for monetary gain instead. Although in the game failing to get the information about Dandelion for this reason seems to be attributed to incompetence, rather than malice, I guess the latter is just less in character for the witcher.
Edit: returning to the topic of Dijkstra's reward and specifically his henchmen in A Poet Under Pressure, they are apparently no longer available in the current version of the game if the treasure is not found. I am quite sure I got them in the past without the treasure, and the old save mentioned earlier also seems to confirm that, so I think this is something that was changed in one of the patches.