This article's content is accurate to the most recent public game version, 0.4.8d.
"Isabelle saps an enemy's mana reserves for a net gain of 20MP, and 10 MP from ambient mana."
Cost: 10 MP
Source: Ganguro Isabelle - Starting skill.
Negative Charge is a spell that drains 30 MP from a single enemy target for the user, and additionally restores an additional 10 MP on top.
For further clarification, this spell costs 10 MP, the enemy loses 30 MP which Isabelle gains, and Isabelle gains a further 10 MP from "ambient mana". This results in a net gain of up to 30 MP for Isabelle (with a corresponding loss to the enemy), provided the enemy has enough MP to drain, and the 10 additional MP is used to offset the initial cost.