If it's ok to set
boostReconcile.InvoiceDetail
to its default value (
null
if it's a reference type) when there is no InvoiceDetail with a matching IMEINumber then you can do this instead:
boostReconcile.InvoiceDetail = InvoiceDetails.FirstOrDefault(x => x.IMEINumber == boostReconcile.BoostSheetLine.BoostImei);
If there should ever be only 0 or 1 InvoiceDetail that matches this criterion, you should use SingleOrDefault() instead:
boostReconcile.InvoiceDetail = InvoiceDetails.SingleOrDefault(x => x.IMEINumber == boostReconcile.BoostSheetLine.BoostImei);