Upgrade: Processing the Upcoming Invoice

Hey! In case we have a plan that has price ranges (depending on the units). Imagine a user who has a subscription with 150 units. He wants to modify his subscription and take only 100 units but for the same plan (or else for another plan with units, or just change plan for another plan with a fixed price, without quantity), in this case we must specify the quantity in our function, what that it happens.

So, should I have something that looks like this:

public function getUpcomingInvoiceForChangedSubscription(User $user, SubscriptionPlan $newPlan, $quantity = 1)
    {
        
        $stripeCustomer = $user->getStripeCustomer();
        $data = 
        [
            'customer' => $stripeCustomer->getStripeCustomerId(),
            'subscription' => $stripeCustomer->getSubscription()->getStripeSubscriptionId(),
            "subscription_proration_behavior" => "always_invoice",
        ];
            
        $stripeSubscription = $this->findSubscription($user->getStripeCustomer()->getSubscription()->getStripeSubscriptionId());
        
        if($newPlan->getPlanId() == $stripeCustomer->getSubscription()->getPlan()->getPlanId())
        {
            
            $subscriptionItems = [
                [
                    "id" => $stripeSubscription->items->data[0]->id,
                    'quantity' => intval($quantity),
                ]
            ];
        }
        else
        {
            $subscriptionItems = [
                [
                    "id" => $stripeSubscription->items->data[0]->id,
                    "deleted" => true,
                ],
                [
                    "plan" => $newPlan->getPlanId(),
                    "quantity" => intval($quantity),
                ]
            ];
        }

        $data["subscription_items"] = $subscriptionItems;

        dump(\Stripe\Invoice::upcoming($data));
        return \Stripe\Invoice::upcoming($data);
    }

Now, in my controller, on the <b>previewPlanChange function</b>

            $stripeInvoice = $stripeClient->getUpcomingInvoiceForChangedSubscription($this->getUser(), $plan, $quantity);
    
            $currentUserPlan = $this->getUser()->getStripeCustomer()->getSubscription()->getPlan();
    
            /* contains the pro-rations *plus* - if the duration matches - the next cycle's amount */
            $total = $stripeInvoice->amount_due;
    
            if($plan->getDuration() == $currentUserPlan->getDuration())
            {
                $total -= $plan->getPrice() * 100;
                dump($total);
            }
    
            return new JsonResponse([
                "trial" => false,
                "total" => $total/100
            ]);

It works if I increase the quantity. On the other hand, if I decide to decrease the quantity, then, systematically, the "amount_due" will be equal to 0 while the "total" will have the good value taking into account the prorations.

The point is if I change :<b>$total = $stripeInvoice->amount_due;</b> by<b>$total = $stripeInvoice->total;</b>
I'm not sure if it's "correct".

Note that I only have this problem in the preview of the invoice. If I confirm, Stripe will do the prorations, the calculations according to what I have to pay, what I have left in my balance, etc. The problem really lies in the preview of the invoice when changing plans