What if during transaction Ripple price drops, in this case the receiving bank will get less amount
The market makers (which can be the banks themselves) price this slight risk into the rates they offer. Most of the time, this doesn't happen, and they make a slight profit. When it occasionally does happen, they may take a slight loss. Fortunately, the price can't move very much in so little time -- and, if necessary, the amount of time can be brought down to sub-second levels by using off-chain scaling schemes such as payment channels.