You can use the attached code to specify wallet currency.

add_filter('woo_wallet_wc_price_args', 'woo_wallet_wc_price_args_callback');

if(!function_exists('woo_wallet_wc_price_args_callback')){
    function woo_wallet_wc_price_args_callback($args){
        $args['currency'] = 'USD';
        return $args;
    }
}