The gain on sale of the Iraqi Dinars is going to be taxed as ordinary income under §988 of the internal revenue code.
Thus the income is not taxed as capital gain which could potentially be taxed at 15% if the underlying property were held for greater than one year.
Ordinary income is taxed at your highest marginal tax rate. (i.e. the same tax rate as wages or rental income)
So there really isn't anything that you can do to lower your tax rate on gain on sale of foreign currency.
I hope I have provided the clarity you were looking for. Please let me know if you have any further questions.