Praise be to Allaah.
If a person owns wealth
that reaches the nisaab (minimum threshold) and one full year has passed, he
must pay zakaah on it, whether this wealth is cash, gold or silver, from the
mahr (dowry) or anything else, whether he is saving it for a trip or to
build a house or any other purpose.
The nisaab is the
equivalent of 85 grams of gold or 595 grams of silver.
Based on that, if your mahr
that is kept in the bank is equivalent to the nisaab or more, then you must
pay zakaah on it every time one hijri year passes, so you must pay
one-quarter of one-tenth of it (2.5%).
We must also point out that
it is not permissible to keep money in riba-based banks except in cases of
necessity or fear, to protect it, if there are no Islamic banks, so long as
there is no interest involved.
And Allaah knows best.