Negative values in ecommerce?

We have a site where visitors can deposit funds and can buy stuff, but they can also withdraw back to their bank.

So, if I define withdrawal as an ecommerce item and track it, it needs to have negative value as funds leave our account.

However, when I send a negative number as value in “doTrackGoal”, the item does not get tracked at all. Where is this value checked for being negative, and how can I change it to record negative values?

Please contact:
for more info as this is non trivial