question about some fundamentals of forex

 

My question is a bit fundamental

suppose that I have a forex account with 1000 USD dollars in balance.

Now suppose that I want to trade in the EUR/USD pair, we know if the price will go up you should by EUR and sell USD, then do the opposite (sell EUR and buy USD) at a higher price to make a profit.

But if you know that the price will go down you will do the opposite, you will but USD and sell EUR, and at a lower price you will do the opposite (sell USD and buy EUR) and here is the catch

1- how can you buy USD and you have already USD in your balance " How can you buy dollars for dollars" ??!

2- (Assuming the price did went down) Suppose you bought only with 100 USD leaving 900 USD in the balance, why the value of the 100 USD that you bought have increased while the 900 USD in the balance did not change although both are in USD "the change in the EUR/USD did not affect both of them only the one you were trading with" ?

Reason: