As the actual Independence Day is not until March 2016, Scotland as part of the UK till then will still be in the EU. The current Scottish Government and opinion polls in Scotland have expressed the wish to remain in the EU. It is hoped that negotiations would be completed within the 18 months and Scotland would then remain in the EU. The current EU president has stated that iScotland would be a 'special case' NOT a new country seeking admission. It is inconceivable that a pragmatic EU would 'throw out' a nation with the biggest oil stocks of any EU country and the most extensive fishing grounds.
To join the Euro takes time as there are conditions, including that the member state must have a Central Bank, so Scotland could not join on day one even if it wanted. If Scotland cannot negotiate a currency Union with rUK (the preferred option) then they will almost certainly still use the pound, informally, at least in the short term. In either of these cases the pound would have the same value whether it was a rUK pound or a Scottish pound. If in time, Scotland sets up its own Central Bank and own currency, not tied to sterling, then, of course, there would be an exchange rate, just as there is at present between the £sterling and the Euro, Dollar, Kroner, etc. and trade would continue across borders and currencies just as it does now throughout the world. HTH