Why did I receive foreign currency after trading a foreign Future contract?

When you open a margin account you have the ability to borrow from IBKR. You can not set a limit or a set only a specific amount of margin to use in a margin account. If the account has sufficient Available Funds then a new order would be accepted. There is a calculated margin requirement for positions in a margin type account displayed as an Initial margin and Maintenance margin requirement. In addition to contracts traded in the account any borrowed cash from Forex trading or from trading outside the account’s Base Currency can have a calculated margin requirement. There are tools available so you can monitor your account’s margin requirements and cash balances in real time or as a daily snapshot…

It is possible for an account holding a Future Option position to have positive Available Funds due to the position's Unrealized P&L yet still hold a negative cash balance. In this example, the account would not be allowed to Buy additional Future Option contracts to add to that position as there would be no cash to pay the seller. Future and Future Option contracts have no loan value, so IBKR does not create a loan against commodities…

The only way to avoid currency risk is to never trade outside the account's base currency. For example, if you have a margin account and trade a foreign security without converting funds prior your account will take on a short cash balance for the amount of the trade. This cash position will carry currency risk, of which you can view at the end of each day in the FX Translation Gain/Loss line in the Cash Report section of your Activity Statement. Please note, IBKR does not give any kind of trading advice and you need to decide how to manage any currency risk when trading in contracts denominated in a non-base currency or when holding non-base currency balances…

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. …