var results = from p in pending
              group p by p.ContactID into g
              let amount = g.Sum(s => s.Amount)
              select new PaymentItemModel
              {
                  ContactID = g.ContactID, // <--- Error here
                  Amount = amount
              };

SELECT
    lp.PurchasedFromContactID,
    SUM (lp.Amount)
FROM
    LeadPurchases lp
GROUP BY
    lp.PurchasedFromContactID

from p in pending
              group p by p.ContactID into g
              let amount = g.Sum(s => s.Amount)
              select new PaymentItemModel
              {
                  ContactID = g.Key,
                  Amount = amount
              };

group p by new
{
    p.ContactID,
    p.Field2,
    p.Field3
}into g
select new PaymentItemModel()
{
    ContactID = g.Key.ContactID,
    anotherField = g.Key.Field2,
    nextField = g.Key.Field3
};