To answer what retention monies are:
As I understand it, it is money that is withheld from you as you leave and is used to cover any unforeseen expenses like EK accommodation having to do some extra cleaning of your villa/apartment or changing the garden back to original state, or perhaps a last etisalat/du phone bill that wasn't quite paid in full etc.
I've been told this amount is about 4000 dirhams. But you are supposed to get it back at some point.. have heard as quick as a few weeks to several months as one poster mentioned above.
During the out process checklist, you will be asked to put a bank account as to where you want these retention monies sent after you've left. I just gave them my bank account number in my home country.
K