While querying the invoice/estimate from QBO, we're not able to see discount line detail where we need to apply our discount on the invoice/estimate header. As we're not able to see discount line detail, we cannot update discount amount or % in QBO invoice/estimate.
QueryService<Estimate> querySvc = new QueryService<Estimate>(serviceContext); List<Estimate> quickBookEstimate = querySvc.ExecuteIdsQuery("SELECT * FROM Estimate").ToList();
Let me get you to the right support team that currently handles QBO API concerns. Aside from being an expert in handling QBO API app integration, our Intuit Developer Team can help update the discount line detail on your invoice and estimate.