WooCommerce: Add-on option label doesn’t take negative price

I’m using WooCommerce v. 2.1.7. I’m also using the following skinning plugin, which made my skinning customizations easier:
http://themeforest.net/item/mommerce-modern-multipurpose-woocommerce-theme/6727886

I have a product page that looks something like this:

I’m trying to add a price to “Try on” a dress for less than the 7-day rental price ($910). It’s set up like the following in the WooCommerce backend:

When I try to select “New Option” and add a -875 or any negative number, WooCommerce screams at me. How can I get this to accept a negative value?

Ideas I thought of:

1. Where do I find, and remove, the validator that makes WooCommerce scream at me for entering a negative value?
2. Where exactly do I find the code in this system to hard code the try-on price value?