Your post is too vague to give you a proper solution. Chaosium's suggestion works but I'm certain it's not what you want (we can only guess).
What is 50% mana? Is that current mana or max mana? Do you need more than 50% current mana to use the item in the first place?
What does exchange mean? Is it a 1:1 ratio? If 50% of my current mana is equal to 200 mana, does that mean I gain 200 life?