Minting gold coins and spread them in the IS would not work, the extra coins would be just as good as the islamic coins. Making coins that looks like gold is not easy, and can be detected by anyone.
They don't have to forge any, just put the word out they or a vast amount of fakes made out of ex projectile DU and brass, to poison you if you try biting one, or to irradiate your nuts when they're in your pocket and sterilise you.