Minimum Award Amount in order to Purchase Additional Miles?

Hi there,

Currently on the phone with United after booking a saver economy (12.5K miles) one way LAX-EWR for this Sunday. Ticket did not issue properly last night after I called to purchase the additional miles needed (about 3.5K) to balance out my account. Agent is telling me the base amount of the award flight has to be to purchase the additional miles is 20K(?). However, agent last night was happy to purchase the additional miles for me over the phone...

Is this agent correct? I'm having a hard time finding proof of this on the website...

What annoys me in this situation is not so much the policy (it is what it is) but the fact an agent was able to purchase miles for me without knowing the policy...

Thanks