You have a 100000 margin account at IB with no current trades on. On Monday, coffee is trading at 100. You go long one contract. By Friday of the same week, has moved up to 105 and you get out for a 5 pt profit. At anytime during this week, were you being charged margin interest?
No. There is no borrowing for futures transactions, as you only need the margin requirement (that is, required from the exchange) to be able to enter into that position. You may be charged a rollover or daily position fee ($0.10 I think?) when you hold overnight like the above says.