Setting a currency just after login

functions.php

<?php 

// the code below goes in your functions file

function my_custom_pos_template_redirect(){
  if ( isset( $_SESSION['wcj-currency'] ) ) {
    $_SESSION['wcj-currency'] = 'USD';         // force session variable to correct currency
    // unset( $_SESSION['wcj-currency'] );     // or - unset the variable altogether
  }
}

add_filter('woocommerce_pos_template_redirect', 'my_custom_pos_template_redirect' );