A long or short currency balance does not offset a margin requirement for a Future contract. Currency can be converted and traded in your IBKR Universal account's Securities segment while Futures contracts are held in your IBKR Universal account's Commodities segment. You will need to have enough Available Funds to meet the calculated margin requirement for any open Futures contracts in addition to any calculated margin requirement for an open Forex currency balance held in the account.
For more information, please visit the Margin page on IBKR's website.