public async Task<RefundResult> InitiateRefundAsync(
    Guid paymentId,
    decimal? amount = null,  // null = повне повернення
    string? reason = null,
    CancellationToken ct = default)
{
    var payment = await _db.Payments
        .FirstOrDefaultAsync(p => p.Id == paymentId, ct)
        ?? throw new NotFoundException($"Payment {paymentId} not found");

    // Перевіряємо, що повернення взагалі можливе
    if (payment.Status != PaymentStatus.Settled
        && payment.Status != PaymentStatus.PartiallyRefunded)
    {
        throw new InvalidOperationException(
            $"Cannot refund payment in status {payment.Status}. " +
            $"Only Settled or PartiallyRefunded payments can be refunded.");
    }

    // Визначаємо суму повернення
    var refundAmount = amount ?? payment.Amount;

    if (refundAmount <= 0 || refundAmount > payment.Amount)
        throw new ArgumentException("Invalid refund amount");

    // Ідемпотентний ключ для refund
    var refundKey = $"refund_{paymentId}_{refundAmount}_{DateTimeOffset.UtcNow:yyyyMMddHH}";

    // Делегуємо провайдеру
    var provider = _providerFactory.GetProvider(payment.Provider);
    var result = await provider.RefundAsync(
        payment.ProviderTransactionId!, refundAmount, ct);

    if (!result.Success)
    {
        throw new PaymentRefundException($"Refund failed: {result.ErrorMessage}");
    }

    // Оновлюємо статус платежу
    var isFullRefund = refundAmount == payment.Amount;
    payment.Status = isFullRefund
        ? PaymentStatus.Refunded
        : PaymentStatus.PartiallyRefunded;
    payment.UpdatedAt = DateTimeOffset.UtcNow;

    // Зберігаємо запис про повернення
    _db.Refunds.Add(new Refund
    {
        Id = Guid.NewGuid(),
        PaymentId = paymentId,
        Amount = refundAmount,
        ProviderRefundId = result.RefundId,
        Reason = reason,
        CreatedAt = DateTimeOffset.UtcNow
    });

    await _db.SaveChangesAsync(ct);

    return result;
}

public class Refund
{
    public Guid Id { get; set; }
    public Guid PaymentId { get; set; }
    public Payment Payment { get; set; } = null!;

    public decimal Amount { get; set; }
    public string? ProviderRefundId { get; set; } // ID повернення від PSP
    public string? Reason { get; set; }
    public DateTimeOffset CreatedAt { get; set; }
}

group.MapPost("/{id}/refund", RefundPayment)
    .WithName("RefundPayment")
    .WithSummary("Ініціювати повернення коштів");

private static async Task<IResult> RefundPayment(
    Guid id,
    [FromBody] RefundRequest? request,
    PaymentService paymentService,
    CancellationToken ct)
{
    var result = await paymentService.InitiateRefundAsync(
        paymentId: id,
        amount: request?.Amount,
        reason: request?.Reason,
        ct: ct);

    return Results.Ok(new
    {
        RefundId = result.RefundId,
        Message = result.Success ? "Повернення ініційовано" : result.ErrorMessage
    });
}

public record RefundRequest(decimal? Amount, string? Reason);