Recast on any champion does not consume mana but requires the amount to recast.
I have tested this on Rakan and Tahm Kench. If you cast a spell that requires a recast, if the initial cast makes you fall below the initial cast mana cost, you will not be able to recast. The mana is not consumed upon recast, but it cannot be reactivated.
Rakan E Tahm Kench W