Glitch your way to success, I can attest that this still works on the PS4 version, I have a GB account but I can't imagine any variation between regions.
So here's the steps in text;
1 - Go to Nulan Village, stand near the shop guy but don't open the shop menu.
2 - Open your weapon tab.
3 - Equip another weapon with Toan and then equip your dagger.
4 - Talk to the shop guy to open the shop menu.
5 - Scroll all of they way down in your item menu.
6 - Put the cursor over the ice attachment in the shop menu.
7 - Press right on the D-pad and X at the same time.
If done correctly you can get the broken dagger which will super boost any weapon it's attached to.
This can be done repeatedly, if you find you're not getting any more broken daggers simply start back at step 2 and get as many as you like.
To get money from this simply attach the broken dagger to any simple weapon, cheap kitchen knives can be bought from Jacks' Store in Queens for 400 and sell for around 16000.
This game went all inception on our asses and gave us a glitch within a glitch. I'm not sure what the variation is here, maybe it's because I entered a dungeon with the dagger equipped and still had the broken dagger attachments in my inventory, or I stopped equipping another weapon and just kept equipping the dagger or if removing the dagger attachment from 1 weapon and later putting it on another weapon caused the attachments to become weird and lose much of their benefits.
My suggested approach to avoid the glitch is to do the steps exactly as listed and to only equip a dagger attachment to one weapon, meaning as soon as it's on a weapon leave it there, if you want other weapons with the attachment boost just make some new daggers.