How to handle cases where parent Eloquent model does and does not exist – Laravel

Given a Dues and Payment Laravel Eloquent models with the following defined relationships:

    /**
     * Get the payment(s) for this set of dues
     */
    public function payments()
    {
        return $this->hasMany(Payment::class);
    }

    /**
     * Get the Dues to which this payment belongs.
     */
    public function dues()
    {
        return $this->belongsTo(Dues::class, 'dues_id'); // custom key because Laravel assumes no s on model name
    }

where more than one Payment can be assigned to a Dues model (i.e. broken into multiple payments), I am trying to create a Payment and associate it to the Dues model in my controller, and address the case where the Dues model may or may not exist. My thought is to use Eloquent methods to find an existing model if it exists"

    public function store(StorePaymentRequest $request, User $user)
    {
        $validated = $request->validated();

        $calculator = new DuesRenewalCalculator($validated['user_id']);
        $year = Carbon::now()->year;

        $dues = Dues::firstOrNew(
            ['user_id' => $validated['user_id'], 'year' => $year],
            [
                'year' => $year,
                'type' => 'renewal',
                'member_dues' => $calculator->dues,
                'building_assessment' => $calculator->buildingAssessment,
                'work_hours_assessment' => $calculator->workHours,
                'nra_dues' => $calculator->nra,
                'associate_dues' => $calculator->associate,
                'junior_dues' => $calculator->juniors,
                'total' => $calculator->total
            ]
        );

        $dues->user()->associate($user);
        $dues->save();

        $payment = new Payment([
            'payment_date' => $validated['date'],
            'amount' => $validated['amount'],
            'refund' => $validated['refund'],
            'reference' => $validated['reference'],
            'comments' => $validated['comments'],
            'mode' => $validated['mode']
        ]);

        $payment->dues()->associate($dues);
        $payment->save();

        return Redirect::route('user.dues.index', ['user' => $validated['user_id']])->with('success', 'Work Hours saved');
    }

This code works perfectly if both models are new, but if $dues already exists, I get an error that Column dues_id cannot be null. on $payment->dues()->associate($dues);. Is this because associate() only works when the parent model doesn’t exist?

If i try

$dues->payments()->save($payment);

I get

Column not found: 1054 Unknown column 'dues_dues_id' in 'field list'     

What is the easiest way to handle both possibilities (that the parent Dues model does or doesn’t exist)?