QuickOrder.cs

public class QuickOrderHandler2 : IRequestHandler<QuickOrderCommand>
{
    private readonly IOrderRepository _orderRepository;
    private readonly ISalesProductRepository _salesProductRepository;

    public QuickOrderHandler2(IOrderRepository orderRepository, ISalesProductRepository salesProductRepository)
    {
        _orderRepository = orderRepository;
        _salesProductRepository = salesProductRepository;
    }

    public async Task Handle(QuickOrderCommand request)
    {
        using var trx = _orderRepository.BeginTransaction(IsolationLevel.Serializable);

        var salesProduct = await _salesProductRepository.SalesProduct(request.Sku, trx);

        var quickOrder = salesProduct.CreateQuickOrder();
        _orderRepository.Save(quickOrder, trx);

        await _salesProductRepository.Save(salesProduct, request.Version);

        trx.Commit();
    }
}

public class SalesProduct
{
    public Sku Sku { get; protected set; }
    public decimal Price { get; protected set; }
    public int AvailableToPromise { get; protected set; }

    public SalesProduct(Sku sku, decimal price, int availableToPromise)
    {
        Sku = sku;
        Price = price;
        AvailableToPromise = availableToPromise;
    }

    public QuickOrder CreateQuickOrder()
    {
        AvailableToPromise = AvailableToPromise - 1;
        return QuickOrder.Factory(this);
    }
}

public class QuickOrder
{
    public Sku Sku { get; }
    public decimal Price { get; }

    public QuickOrder(Sku sku, decimal price)
    {
        Sku = sku;
        Price = price;
    }

    public QuickOrder(SalesProduct product)
    {
        Sku = product.Sku;
        Price = product.Price;
    }

    public static QuickOrder Factory(SalesProduct salesProduct)
    {
        if (salesProduct.AvailableToPromise <= 0)
        {
            throw new InvalidOperationException("Product has no quantity on hand.");
        }

        return new QuickOrder(salesProduct);
    }
}