How can I have a margin requirement when I still have some of my own cash?

Current Available Funds is what you have available to open a new position. In the Securities segment of your account, the Available Funds is the Equity with Loan Value less the total Initial Margin Requirements (ELV - Initial margin). For accounts that have been designated as a Pattern Day Trading account, the Available Funds in the Securities segment is the lesser of either today's Equity with Loan Value or the end of the previous day's Equity with Loan Value at 16:15 EST, less total Initial Margin Requirements. The Current Available Funds to open a future contract is the Net Liquidation Value in the Commodities segment less the total Initial Margin Requirements. Upon submission of an order, a check is made against real-time available funds for all open orders plus the order being submitted.  If Available Funds after the order request would be greater than or equal to zero, the order is accepted. If Available Funds would be negative, the order is rejected…

IBKR adds House margins to the Initial margin requirement for a contract. If you hold a position that has a higher Initial margin than Maintenance margin, it is possible for your Available Funds to be negative while your Excess Liquidity is positive. Buying Power is projected as the current Available Funds (Equity with Loan Value less Initial Margin) times a multiple, so when your Available Funds is negative you will not be allowed to enter a margin increasing order…

We will calculate and display a margin requirement on the entire portfolio of positions held in an account even if the account holder has paid for the positions in full and is not borrowing any funds to support them. This is necessary in order to compute the…

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…