First, correct a small mistake. You are trading foreign exchange spot, not futures, so there is no contract problem.

The so-called inventory fee, also called overnight fee, is caused by the spread of the currency you trade between the two countries, so there will be positive and negative situations. For example, the interest rate of the Australian dollar against the US dollar is very high, so doing more in Australia and the United States is positive interest, and shorting in Australia and the United States is negative interest.