It's like transmission oil, but not so oily.
Differential: An amount of money that compensates for the difference in costs (e.g., housing, goods and services) between the home and assignment locations.
And then, when you get there, the home office calls and tells you they rethought it all, and you're coming out ahead by being in Elbonia, so they decided to eliminate the differential. And you don't have enough money to go back home, so you're stuck for two years.