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.

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.

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.