The upgrade for cash is processed differently. LH changes the booking class and seats the pax in a different cabin. The underlying ticket is unchanged. Instead an EMD is issued for the upgrade. So UA doesn't have to do anything with the underlying ticket at all. All LH does is issue an EMD and records that on their end. The LH agent can see the EMD when you call and tells you all is OK. Because it is. The UA agent probably doesn't know how the cash upgrade is processed so even though they can see the EMD they don't see everything as things are not mirrored/mapped into their SHARES GDS.

If you want to be sure, call LH and ask for the ticket number of the EMD which they issued to collect the 700$.