>I tried that Viv before posting and it didn't work as expected, it returned the largest invoice first then the rest in what appeared to be in random order.

Hmm. Should work (I'm assuming you had the OrderBy before the GroupBy). Simple test using:

public class Invoice
    {
        public double invoiceamount { get; set; }
        public int clientid { get; set; }
    }

const double invoiceFilterAmount = 100;

IEnumerable<Invoice> invoices = new List<Invoice>()
                {
                    new Invoice() {invoiceamount = 123.34, clientid=1},
                    new Invoice() {invoiceamount = 19.24,  clientid = 2},
                    new Invoice() {invoiceamount = 47.24,clientid = 2},
                    new Invoice() {invoiceamount = 1000.00, clientid = 1},
                    new Invoice() {invoiceamount = 122.56, clientid = 2},
                    new Invoice() {invoiceamount = 99.99, clientid = 1},
                    new Invoice() {invoiceamount = 100.00,clientid = 2},
                };

            var query = invoices.Where(x => x.invoiceamount <= invoiceFilterAmount)
                                 .OrderByDescending(x => x.invoiceamount)
                                 .GroupBy(x => x.clientid);

May not be super efficient since it's sorting all invoices rather than just those within each group - but I don't think you can do that without anonymous types.