I used to regularly leave Argentina with over 20k. I would tuck it into the pages of a book. I didn't declare leaving Argentina, but did declare coming into the US. They make you fill out a form, and off you go. If the money was originally made in the US, it shouldn't affect your taxes. If made in Argentina, it will not be taxed in the US because the tax rate in Argentina is way higher. However, if you have investments in the US, the US now knows you have additional income sources so you will be in a higher bracket for US income.