Is it permissible to give the mahr in a currency other than that mentioned in the marriage contract?

If the couple agree on giving the mahr in a currency other than that which is mentioned in the marriage contract, that is permissible, so long as the exchange is done at the rate on the day on which it is given, not the rate on the day of the contract, and the equivalent amount is given in full; they should not separate when there is still something outstanding that the husband has to pay. 

The basic principle concerning that is the report narrated by Abu Dawood (3354) and al-Nasaa’i (4582) from Ibn ‘Umar (may Allaah be pleased with him) who said: I used to sell camels in al-Baqee’, and I would sell them for dinars and take dirhams, and I would sell them for dirhams and take dinars. I asked the Messenger of Allaah (peace and blessings of Allaah be upon him) about that and he said: “There is nothing wrong with taking the price of that day, so long as you do not separate when there is still something to be paid.” Classed as saheeh by al-Nawawi in al-Majmoo’ (9/330), Ibn al-Qayyim in Tahdheeb al-Sunan, and Ahmad Shaakir in Tahqeeq al-Musnad (7/226). Classed as da’eef (weak) by al-Albaani in Da’eef Abi Dawood. The hadeeth is in accordance with shar’i principles on buying and selling and on riba (usury), hence the fuqaha’ adopted it. See: al-Sharh al-Mumti’ (8/305). 

The Standing Committee for Issuing Fatwas was asked about a man who took a loan in French currency, and when he paid it back, the lender asked him to pay it in Algerian currency. 

They replied: 

It is permissible to pay it off in Algeria in the same amount of French currency, or in the equivalent amount of Algerian currency at the rate of exchange on the day when it is paid off, provided that it is paid before separating. End quote. 

Fataawa al-Lajnah al-Daa’imah (14/143). 

And Allaah knows best.