All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
In principle, Mahr should be paid in gold, as agreed. If that wasn`t possible, some of the Shaffii jurists have allowed the husband to make other forms of payment, such as banknotes, but with conditions:
First: No pre-agreement was reached as to the form of payment, and that the form of payment was agreed on only on the day of payment. This is because agreeing on paying in banknotes instead of gold without doing that on the spot leads to Riba Annase`ah (Increasing the installment in return for delay in payment) since it becomes an exchange contract where immediate and on the spot payment is a condition.
Second: Payment should be made according to the price of gold on the day of settling the debt(Mahr), not on that of paying the Mahr (Date of concluding the marriage contract).
Third: The whole sum should be paid based on what is mentioned earlier, where payment on the spot is a condition in the exchange contract.
Al-Khateeb Ash-shirbini(May Allah have mercy on him) said: "It is permissible to settle a debt in a different form of payment, even if that debt wasn`t in cash. This is based on the narration of Abdullah ibn Umar who said: "I used to sell camels at al-Baqi for dinars and take dirhams for them, and sell for dirhams and take dinars for them. I would take these for these and give these for these. I went to the Messenger of Allah (PBUH) who was in the house of Hafsah. I said: Messenger of Allah , take it easy, I shall ask you (a question): I sell camels at al-Baqi'. I sell (them) for dinars and take dirhams and I sell for dirhams and take dinars. I take these for these, and give these for these. The Messenger of Allah (PBUH) then said: There is no harm in taking them at the current rate so long as you do not separate leaving something to be settled."{Muslim}, (Moghni Al-Mohtajj, 2/464). Resolution No.{75/ 6-8) of the Islamic Fiqh Academy stated: "It is permissible for the creditor and the debtor to agree, on the day of settlement, on settling the debt in a different currency."
In conclusion, if the husband wishes to pay the Mahr in banknotes, then it isn`t permissible to do so by installments, and he may not delay paying any portion of it. If he wasn`t capable of that, then the debt remains in gold; however, he can pay any portion of it in cash based on the price of gold on the day of settlement. And Allah knows best.