It's not quite a loan. It's more like a lease. The zoo paid a lot of money to have these two pandas (unlike Ling-Ling and Hsing-Hsing, who were given to the zoo by the Chinese during the Nixon Administration; times have changed and China doesn't give away pandas anymore).
I think the contract was to expire this year, but perhaps if the remaining cub survives that might be a reason to extend the deal.
Edited to add: The zoo's website says the current agreement runs through this December 5.