Question

I was wonderig how come money is used as a medium of deffered payment and wealth accumulation. That is if someone have 100 unit of currency today, it won't be worth the same: that is to say it wont buy same amount of goods in future?

Answer 1

I think that's why interest exists, to compensate for the loss of value and possible profits/

Answer 2

But interest rates does not always correspond to rise in prices (inflation). Or they do? can i have a case study?

Answer 3

The reason money is used as a method of deferred payment and wealth accumulation is because of it's liquidity. At any given point of time, once you have the money, you are free to exchange this money for any other good or service (or another currency) wherever your money is considered to be legal tender. Barring bonds and fixed-term loans, this is generally true of all money. As a rather crude example - try buying something today at a shop for $10, and then try the same with $10 worth of gold. Money is simply far more universal - and can easily be invested if the opportunity cost of keeping it on hand seems too high for you.
If you are referring to money that has been promised to a seller of a good in exchange for the good, to be paid later - this is usually accepted by the seller due to a double mark-up on the cost of the good - one for the fact there is inflation that needs to be counteracted (which you've mentioned), and another as a premium on the risk which is suffered by the seller of the good (the chance that the buyer will not go through with his payment).