I believe I can clarify.
The minimum foreign currency exchange commission is US$2 (plus GST if you're an Interactive Brokers Singapore Pte. Ltd. customer), and this is also the maximum charge if your order is executed as a single trade and the amount isn't "huge." Let's assume for a moment you have a cash (non-margin) account. Here are two scenarios:
1. You have set your account's base currency to U.S. dollars. In this case you'll be charged the exact U.S. dollar commission. If you don't have enough U.S. dollars in your account to cover the commission then Interactive Brokers will grab the commission from available cash in another currency (such as Singapore dollars) at the current spot conversion rate.
2. You have your account's base currency in Singapore dollars. In this case Interactive Brokers will use the current spot rate to determine the commission in Singapore dollars, then grab the commission from your Singapore dollars. If you don't have enough Singapore dollars Interactive Brokers will grab the commission from another available cash currency, such as U.S. dollars.
Obviously you cannot convert currencies to the exact penny, or at least it's very difficult. That's OK. As long as the residual amount is less than US$5, Interactive Brokers will eventually (within a couple days usually) automatically convert residual non-base currency cash into your base currency, at spot rate and without a commission.
If you have a margin account then you should watch out for negative cash positions, although these aren't expensive in today's low margin interest rate environment.