Hi Mervin,

I believe the following code should do it:

if(current_user_can('customer')) {
    function remove_add_to_cart_on_0 ( $purchasable, $product ){
        if( $product->get_price() == 0 )
            $purchasable = false;
        return $purchasable;
    }
    function check_user_is_agent_with_access(){
        // check if switch cookie is set
        if (isset($_COOKIE['salesking_switch_cookie'])){
            $switch_to = sanitize_text_field($_COOKIE['salesking_switch_cookie']);
            $current_id = get_current_user_id();
            if (!empty($switch_to) && is_user_logged_in()){
                // show bar
                $udata = get_userdata( get_current_user_id() );
                $name = $udata->first_name.' '.$udata->last_name;
                // get agent details
                $agent = explode('_',$switch_to);
                $customer_id = intval($agent[0]);
                $agent_id = intval($agent[1]);
                $agent_registration = $agent[2];
                // check real registration in database
                $udataagent = get_userdata( $agent_id );
                $registered_date = $udataagent->user_registered;
                // if current logged in user is the one in the cookie + agent cookie checks out
                if ($current_id === $customer_id && $agent_registration === $registered_date){
                    return true;
                }
            }
        }
        return false;
    }
    if (!check_user_is_agent_with_access()){
        add_filter( 'woocommerce_is_purchasable', 'remove_add_to_cart_on_0', 99, 2 );
    }
}

Please check and let me know.

Kind regards,

Stefan