You have it backwards ... the limit is on foreign investments not US ones. You were properly taxed at 20% cap gain + the NIT tax of 3.8%. Review the CGQDTW and the form 8960 for the information you seek.
So basically they will allow you a credit on the gain at a rate of no more than 15% ... since you paid more you can show them. If you had paid at a lower rate then they would reduce/limit their credit... does that make sense ?
