It is unlawful to determine or manipulate the price of gold and silver and copper as in what amount of goods they can buy, it is however possible and allowed to determine the difference in value between gold and silver (ie: x dirhams = y dinars) as currency since both are lawful currency, copper can also be included in the equation and any excess mined can easily be released as jewellery and base metal use in goods. If there is a sharp spike in the availability in one currency item over another, extraction and refining would be limited by market difference and cost efficiency, this happens with oil on a regular basis and saudi arabia demonstrated the ability to do this just recently (although that had a lot to do with petty regional squabbles and sabotage in the course of america's divide and conquer strategy in the region and also due to the hammering that the dollar was taking as a result of the output competition). In extreme circumstances a correction could possibly be made.
Caliph 'Umar ibn al khattab demonstrated the will to hold hoarders and market manipulaters to account (read sahih Muslim book of trading for more info, that is a legal issue and not one limited to the circulation of rare base metal currency.
so we can see that there are many lawful and reasonable ways of resolving such minor issues in comparison to the usurer anarchy and corruption brought about by paper and digital currency where prices of goods are manipulated increasingly and even output destruction is practised on occasion sue to the havoc such false currencies wreak on the markets.
The blackmarket excuse doesn't hold water since the government fixing of prices of goods is usually ignored anyway based on supply and demand, this phenomenon is starkly demonstrated during the run-up to and through ramadan.
Also the blackmarket for paper currency is also vast, and for those in control of digital currency it is run like heavily controlled organized crime and a systematic siphoning of wealth from the global community.
The claim that there are more goods than gold and siver indicates a basic lack of understanding of the law of supply and demand - or a willful ignorance of such due to the desire to manipulate the masses and usurp their wealth unlawfully, it just means that less gold, silver and copper would be required to make a purchase of goods, and that the capitalist manufacturing process would reflect supply and demand better in place of consumerist hype and trend setting in order to push useless goods churned out in an unbridled manner.
The Prophet pbuh himself refused to control natural fluctuations whilst they took place in his presence, and has also indicated huge scarcity and high prices of goods in future,
Usury is inherent in paper currency since it is inflated constantly and the prices of goods are raised constantly (when considering annual/decade/century /millenial graphs), until there is a crash - which the usurers themselves call "market correction". And maybe compare the price in goods with gold with the steady decrease in the price of debased metal (starting with the romans i believe) and illusory paper currency over the last one thousand years (you can find historic charts on goldprice.org and on other web resources).
Also the hadith regarding the bull's head and one hundred dinars doesn't indicate a problem with the dinar but the unjust sanctions, sieges and sabotage by people that create scarcity in goods.
To attempt to push the illusion that paper and digital currency is somehow less prone to manipulation than gold and silver is deceptive since both paper and digital concepts fail when standing an honest scientific test in terms of overall stability.
Bear in mind that i am referring to a global order subject to the laws of God and not to the current chaotic hotchpotch of current feifdoms which are on their way out.
the middle eastern leaders will soon realise that they are bickering to weaken themselves in preparation for such an outcome.
To musailamah (the o frog half in water and half in mud, half the fiefdom geezer):
inna al arda li Allah, yoorithuhaa man ya shaa a min 'ibaadih, wal 'aaqibatu li al muttaqeen.
وَلَا تَبْخَسُوا النَّاسَ أَشْيَاءهُمْ وَلَا تَعْثَوْا فِي الْأَرْضِ مُفْسِدِينَ
026:183*Khan:"And defraud not people by reducing their things, nor do evil, making corruption and mischief in the land.
026:183*Maulana:And wrong not men of their dues, and act not corruptly in the earth, making mischief.
026:183*Pickthal:Wrong not mankind in their goods, and do not evil, making mischief, in the earth.
026:183*Rashad:"Do not cheat the people out of their rights, and do not roam the earth corruptingly.
026:183*Sarwar:and do not defraud people in their property or spread evil in the land.
026:183*Shakir:And do not wrong men of their things, and do not act corruptly in the earth, making mischief.
026:183*Sherali:'And diminish not unto people their things, nor go about the earth, creating disorder;
026:183*Yusufali:"And withhold not things justly due to men, nor do evil in the land, working mischief.
Quran ch 26:183
The word used is "laa tabkhasoo" seeming to indicate debasing, diminishing, sabotaging, ruining unlawfully, defrauding etc.
http://www.macrotrends.net/1333/historical-gold-prices-100-year-chart
Notice the huge spikes taking place and increasing with every fiat monetary collapse.
Federal reserve comes in during a crisis and synthetically stabilizes currency after world war 1with all the economic woes blamed on "the great war",1930's when the dollar was being ditched and roosevelt confiscated the gold ammassed after tributes from world war 1, and then had all the woes blamed on world war 2 which was also manufactured, the early eighties huge crash (that's when i come on the scene

), the "bubble burst" of 2000-2001 and the birth of the "war on terror" where bush slows the dollar collapse with an arms race and a huge media distraction during a reign of terror and censorship precedent, and the 2008 crash which was blown beyond recovery levels with unprecedented qe (inflation injection).
notice also that the fiat currency problem is amplifying with every incident and that each incident is being plugged with huge amounts of inflation and only exacerbating the situation while huge global distractions appear as coincidence.