SaleDocs.Cs

public class SaleDocument
{
....
  public decimal GetDiscerpancyAmount(ISpecification<SaleDocument> spec) =>
    spec.IsSatisfiedBy(this) ? ConfirmedPrice - DocumentNetPrice : 0;
....
}

public class SaleDocumentService
{
  public SaleDocumentService(DiscerpancyAmountSpecification spec)
  {...}
  public void CalcAmount()
  {
    saleDocumentObject.GetDiscerpancyAmount(spec);
  }
}