You have to send first.
Public service announcement:
When you trade OTC you engage in a transaction with people you possibly know nothing about. You may send your BTC to the person, and never get anything back. Or you may send your $currency to the person expecting BTC, and get nothing in return. This is a highly undesirable outcome for you, and you should do your best to guard against that. If you do not have a history of previous transactions with this person, or otherwise do not trust your counterparty, there are several mechanisms you can use to mitigate the risk of fraud:
Since fees on Liberty Reserve transactions are reasonable (1%) using a third party escrow works for this. A trusted escrow partner would likely be found on #bitcoin-escrow in which the #bitcoin-otc Web of Trust (WoT) can be used to see the escrow partner's trust history.