How to make a Data Selector a negative value in a rule action?
You have two ways to do this.

Either you use PhP filter, which will add under "data selector" and "add offset" a new option, "php evaluation", which will allow you to post-treat this value. Then you'll just have to set return intval(- $value);

Or you can use whole Rules, by :


Action: Add a value, and select your [node:field-labpoints]
Action: Calculate a value, and make it negative
In your action, use this value instead of the one you selected with the selected.