Essentially, if you short -100,000 EUR/USD you are telling the counterparty that you will provide them with 100,000 Euros.. just not yet. The counterparty in effect is telling you that Euros will increase in value vs. the USD and you are saying the opposite..
You are neither buying nor selling Euros, you are entering into an agreement to sell Euros in exchange for USD only at a later time. You can do this voluntarily when the trade goes against you, or goes in your favor, or involuntarily when you run out of margin..
You will eventually provide the counterparty with 100,000 Euros in exchange for USD
Unlimited shorting is one of the main attractions to FX trading.