Specificationc.s

public class OrderWithoutInvoiceSpecification : Specification<SaleDocument>
{
    ...
}
public class ApprovedOrderWithoutInvoiceSpecification : AndSpecification<SaleDocument>
{
    public ApprovedOrderWithoutInvoiceSpecification()
        : base(new ApprovedOrderSpecification(), new OrderWithoutInvoiceSpecification())
    {}
}
public class WaitingForInvoicesSpecification : AndSpecification<SaleDocument>
{
    public WaitingForInvoicesSpecification() 
        : base(new ApprovedOrderWithoutInvoice(), new HasDiscrepancySpecification())
    {
    }    
}