>>>>>>Off the top of my head, if I understood your problem correctly. It would be something like this:
>>>>>>

>>>>>>var list = (from  d in _dbSet.Where(x => x.Account == accountName && x.Hidden == false)
>>>>>>               from t in transact .Sum(s=>s.extension).Where(w=>w.extension != null && w.invoiceNo == d.InvoiceNo).
>>>>>>              select(x => new InvoicesList { InvoiceNo = x.InvoiceNo, Account = x.Account, Hidden = x.Hidden, 
>>>>>>                                                          Descrip = x.Descrip1, Finalized = x.Finalized, Created = .DateTime}, InvoiceBalance= t)).ToList();
>>>>>>

            var list = new List<decimal?> {2.5M, null, 2.5M, 5M};
            var s = list.Sum();

this returns 10 as the sum which is correct. it is ignoring the nulls... so try this

var list = (from  d in _dbSet.Where(x => x.Account == accountName && x.Hidden == false)
               from t in transact .Sum(s=>s.extension).Where(w => w.invoiceNo == d.InvoiceNo).
               select(x => new InvoicesList { InvoiceNo = x.InvoiceNo, Account = x.Account, Hidden = x.Hidden, 
                                                               Descrip = x.Descrip1, Finalized = x.Finalized, Created = .DateTime}, InvoiceBalance= t)).ToList();