Volume 3, Book 34, Number 302:
Narrated ‘Ali (radiallaahu `anhu):
I got an old she-camel as my share from the booty, and the Prophet had given me another from Al-Khumus. And when I intended to marry Fatima (radiallaahu `anhaa) (daughter of the Prophet), I arranged that a goldsmith from the tribe of Bani Qainuqa’ would accompany me in order to bring Idhkhir and then sell it to the goldsmiths and use its price for my marriage banquet.
Volume 3, Book 34, Number 303:
Narrated Ibn ‘Abbas (radiallaahu `anhu):
Allah’s Apostle (sallallaahu `alayhi wasallam) said, “Allah made Mecca a sanctuary and it was neither permitted for anyone before, nor will it be permitted for anyone after me (to fight in it). And fighting in it was made legal for me for a few hours of a day only. None is allowed to uproot its thorny shrubs or to cut down its trees or to chase its game or to pick up its Luqata (fallen things) except by a person who would announce it publicly.” ‘Abbas bin ‘Abdul-Muttlib (radiallaahu `anhu) requested the Prophet, “Except Al-Idhkhir, for our goldsmiths and for the roofs of our houses.” The Prophet (sallallaahu `alayhi wasallam) said, “Except Al-Idhkhir.” ‘Ikrima said, “Do you know what is meant by chasing its game? It is to drive it out of the shade and sit in its place.” Khalid said, “(‘Abbas said: Al-Idhkhir) for our goldsmiths and our graves.”
We learnt previously that we have to be careful while exchanging goods; one: they have to be of the same kind, two: they have to be of the same amount. For example, good dates for good dates and not bad dates for good dates.
If a person has to exchange one kind of goods with another, the right way to go about it is to sell those goods, and buy the desired goods with the money acquired. And that’s what’s being reiterated here (in the two narrations above). During that time in Arabia, people used to exchange Idhkhir (a kind of grass) with gold. What Ali (radiallaahu `anhu) did for his walima was that he sold whatever Idhkhir he collected to the goldsmiths and used the dirhams/dinars for the banquet preparations.