The Mahr is what a sister asks for from the brother she is considering (if they were to marry).  She could ask for whatever she thinks is suitable for her, if the brother cannot fulfil her request the sister could either lower the value of the Mahr or could put an end to the engagement as a whole.

The Mahr would vary according to time in regards to the amount society recognises as a norm/average.  Her right is to ask for the Mahr, whether her fiance agrees or not, due to whatever circumstance, is entirely upto him.