Try this code as an In-Transaction Data Directive on RMA Head:
foreach (var tt in ttRMAHead.Where(tt => tt.Added() && tt.HDCaseNum != 0))
{
tt.RMAComment = (from row in Db.HDCase where row.Company == tt.Company && row.HDCaseNum == tt.HDCaseNum select row.Description).DefaultIfEmpty("").First();
// Alt Version
//tt.RMAComment = Db.RMAHead.Where(R => R.Company == tt.Company && R.HDCaseNum == tt.HDCaseNum).Select(R => R.Description).DefaultIfEmpty("").First();
}