For some orders, the final price or exchange rate is only known at execution. A small buffer is reserved to prevent the order failing on a minor price or currency movement; anything unused is released immediately after execution.
The buffer is approximately 3% where slippage or currency conversion risk applies, around 5% where both apply, and zero where neither applies. This is also why the estimated cost at order placement may differ slightly from the final amount.
*Stocks are available in selected countries and are currently limited to existing clients.