What are the problems if the values of all the currencies of the world are equalised into a single monetary unit?

Take the case of a single trade between a hypothetical individual in US who imports a car say Toyota from Japan. When the exchange rates were different suppose 1 dollar = 240 yean. The Toyota costing 2400000 yen can be  bought for 10000 dollars. Now say the equalisation take place and the guy in US now will have to pay 2400000 dollars instead of 10000 dollars to import the car. Where is he going to get that much money in short notice. He asked to raise 230000 dollars overnight where as previous day 10000 dollars would have done the job.

I feel the last sentence above of mine is a long jump. The problem is when currencies are equqted the general nture of individual economies don't get equated along with currencies for example asset prices like US has large free holding spaces and Japan very little. The supply and demand for this pull prices apart and suddenly we see disparity in exchange rates creeping up. There are so many other factors like skill level of Labor in individua; economies, availability of raw materials in each respective economy, the invesment levels, dextirity of labor, the redistribution mechanism etc; which will make the equality factor a temporary phenomenon and different economies with same currency will find the equality fading ie; suppose if we call the common currency X then slowly American X will have larger value than other X's in a transaction or nobody will be able to maintain the particular charecteristics of each economy. This will also tamper with the reward mechansism and reference risks of each economy. Unless each economy try to correct this and equate the reference risks of their respective economy it won't be scientific to artificially equate currencies.

I have to clarify that Economics as a science defines Man and creating him in a Nation State is a learning process. Once this is tampered then it won't be possible for such individuals to maintain the transient advantages happening out of manipulating like equating currencies.

I think simply equating is not possible ,         but a new common currency for all  where you substitute quality with quantity. Regarding common currency I think it is very challenging. Models like euro in a small scale can be considered, india and srilanka. Though bric is planning for a substitute to dollar china itself is worried about dollar loosing value due to t’s huge dollar reserves.

It will bring instability into the system. It can add to the chaos. It also depends on how much of the currency is in existence, and how much that particular country exports and imports. The more exports they have, the higher demand is for their currency, and thus the higher currency value.