lan_kiosk/app/Http/Controllers/PurchaseController.php

<?php

namespace App\Http\Controllers;

use App\Models\Purchase;
use Illuminate\Http\Request;
use App\Models\Customer;
use Illuminate\Support\Facades\DB;

class PurchaseController extends Controller
{
    /**
     * Display a listing of the resource.
     */
    public function index()
    {
        $purchases = Purchase::get();
        return $purchases->toJson();
    }

    /**
     * Show the form for creating a new resource.
     */
    public function create()
    {
        //
    }

    /**
     * Store a newly created resource in storage.
     */
    public function store(Request $request)
    {
        $data = $request->validate([
            'customer_id' => 'required',
            'amount' => 'nullable',
        ]);

        // Look the customer up before writing anything — creating the purchase
        // first would leave a line item behind that no balance ever paid for
        $customer = Customer::findOrFail( $data['customer_id'] );
        $amount = (int) $data['amount'];

        DB::transaction(function () use ($customer, $amount) {
            $customer->amount_left =  $customer->amount_left - $amount;
            $customer->save();

            Purchase::create([
                'customer_id' => $customer->id,
                'amount' => $amount,
            ]);
        });

        return redirect('customer/' . $customer->id);
    }

    /**
     * Display the specified resource.
     */
    public function show(Purchase $purchase)
    {
        //
    }

    /**
     * Show the form for editing the specified resource.
     */
    public function edit(Purchase $purchase)
    {
        //
    }

    /**
     * Update the specified resource in storage.
     */
    public function update(Request $request, Purchase $purchase)
    {
        //
    }

    /**
     * Remove the specified resource from storage.
     */
    public function destroy($id)
    {
        $purchase = Purchase::findOrFail( $id );
        $customer = Customer::findOrFail( $purchase->customer_id );

        // store() takes the amount off this customer's balance, so removing the
        // purchase puts it back on the same balance
        $customer->amount_left =  $customer->amount_left + (int) $purchase->amount;
        $customer->save();

        $purchase->delete();
    
        return response()->json([
            'success' => true, 'message' => 'Purchase deleted successfully'
        ]);
    }
}