I will add this.

If I have -5000ft of cable @ 1$/ft value, my total value is -5000$

If I correct my ''purchase invoice'', my negative inventory is going down but my total value stay the same so my ''unit'' value increase.

I just made a test:

I bring my item to 0$ value and 0$ quantity.  Then I give him back the right quantity with the real paid price and the ''cost'' is back to normal.  Is it the right way to do it?

I'm actually looking at my ''journal general'' and it's seem correct.

Your Service item that sells the CAT6 cable by the case is not updating your inventory, could that be part of your problem?

Also you can set up an inventory item to buy by the meter and sell by the foot ans long as it know the conversion. to do that you would have to create a new inventory item as you can not change the relationship once you have any transaction for that item. You can not sell an item by two different units of measure, to sell a full case you would have to specify how many ft are in the full case, and possibly do a manual discount for full case quantities, depending on what markup you are applying.

It's still best to create new items that have the correct relationship for the conversion between KM / Meters purchased, and feet sold.

Then post inventory adjustments to clear off all the errors and have the correct pricing and costs. 

If you post an adjustment to an existing item that will clear the quantities and cost off to zero, you may be able to change the item to a 'service' item and save it, then change it back to an 'inventory' item and change the relationship.  Using an item that has been modified this way may work, but it will likely prevent adjusting prior invoices, and It may do unusual things in the future, including reports that don't make sense, and even crashing the program.