in forex, there is a bid price and an ask price.


buy from ask price, and sell to bid price.

example:
1.buy from ask price, then closeout using bid price (sell to bid price).
2. sell to bid price, then you closeout using ask price (buy from ask price)


maybe its too complex for you. in other word, when you buy something, you need to sell it to close it.

+1 - 1 =0

if you sell something, you need to buy it to close it

-1 + 1 = 0