i want to send exactly 85 $
Then you'd be better off using a Fiat transfer service like western union or PayPal etc...
Bitcoin is VERY volatile and the "price" is constantly changing... Sit and watch preev.com or the price on an exchange like Coinbase or Bittrex etc for a while. $85 in Bitcoin as I type this is unlikely to be the same as $85 in Bitcoin when you read this.
(Edit: for the record, it change from 0.01548 btc to 0.01552 btc on preev.com while I was typing this message)
At best, you can agree an amount of BTC with the other party in advance and then send them that exact amount of BTC... And they'll just have to accept any variations in the value of that BTC.