How is the Zakat nisab calculated for money saved in Dirhams in the UAE? Is it based on the nisab of the country where one works, or on the nisab of their home country, given that the nisab in their home country is calculated by silver?

If you possess money that reaches the nisab, whether in Syria or the Emirates, or the total of what you own in both countries, then zakat is obligatory upon you for all of it, at a rate of a quarter of a tenth (2.5%). It doesn't matter whether the nisab is reached based on gold or silver. Currencies are added to each other to complete the nisab if each one individually does not reach it. The conditions for this are that you own the nisab and that a hawl (one lunar year) passes over it.