They agreed to give the mahr in gold instead of cash. What is the ruling?
The basic principle is that the man is obliged to give the mahr that he stipulated to the woman. If they agree on something different, or on more or less, then that is permissible, because Allaah says (interpretation of the meaning):
“but if after a Mahr is prescribed, you agree mutually (to give more), there is no sin on you”
Al-Qurtubi (may Allaah have mercy on him) said: “This means, giving more or less in the mahr, for that is allowed when there is mutual agreement, after the mahr has been agreed upon.” (Tafseer al-Qurtubi, 5/235).
Shaykh Saalih al-Fawzaan said: “If she gives him part of it – part of the mahr – there is nothing wrong with that. If she lets him off paying all or part of the mahr, she has the right to do so. Allaah says (interpretation of the meaning):
‘but if they, of their own good pleasure, remit any part of it to you, take it, and enjoy it without fear of any harm (as Allaah has made it lawful)’
This is dependent on agreement from both parties.”
(Fataawa Noor ‘ala’l-Darb, 109).
So it is clear from this that if the woman agrees to this, there is nothing wrong with that because this is her right.
And Allaah knows best.