Your cash balance does not change when opening a CFD position due to the convention used to compute P&L on CFD positions. It is only when the CFD contract is closed that an amount corresponding to its Realized P&L is applied to the cash balance, with a gain increasing the cash balance and a loss decreasing the cash balance.
Additional Information
Disclaimer: CFDs are complex instruments and come with a high risk of losing money rapidly due to leverage. 57.9% of retail investor accounts lose money when trading CFDs with IBKR. You should consider whether you understand how CFDs work and whether you can afford to take the high risk of losing your money.