It sounds from the email that you may be in a hurry to do this but it doesn't say why it is a time is of the essence scenario. I don't think that they can require you to redeem it as a condition of a refi. Are you trying to roll the cost into your new loan?
The application at the taxation website has a formula to determine the amount. If it is not crucial to the refi itself, and if you're not rolling the cost into the new loan, then you should be able to do this anytime you wan to. More information is on the Assessments and Taxation website, at http://www.dat.state.md.us/sdatweb/Groundrent.html.
You can also call them at 410-767-1151.
They have a loan program available for ground rent redemption if you can't get it done in time to roll it into your refi loan. Best bet is to call them or check out the website to get more details. You could also check with your title company for the refi.
Here is a link to the application for ground rent redemption too: http://www.dat.state.md.us/sdatweb/groundrent_appl.pdf.
Advance Realty Direct