A US citizen (and in most cases residents) owes US taxes regardless of where the taxable event occurs.  It doesn't matter if you leave the US and NEVER come back.  The only way to avoid US taxation is to renounce your citizenship (which generally requires you to already have citizenship in another country).  Now if you are asking if you could "get away" with traveling to russia to do cash deals for rubbles and then laundering those funds though a bunch of offshore accounts?   Maybe but that is asking "will I get caught", not "is this legal".  Honestly if it is a small amount of money it probably isn't worth the trouble and if it is a huge amount of money speak to a good offshore lawyer and CPA.  Tax avoidance not tax evasion.