title: Getting rid of wash out rows in R dataframe by group
body: <p>This is what my dataframe looks like:</p>
  
  <pre><code>df &lt;- read.table(text='
      CustomerName    Sales          TradeDate
      John           1000              1/1/2015
      John          -1000              1/1/2015
      John           1000              1/1/2015
      John           5000              2/1/2015
      John          -2000              3/1/2015
      John           2000              3/2/2015
      John           2000              3/3/2015
      John          -2000              3/4/2015
      John           2000              3/5/2015
      John           2000              3/6/2015
      John          -3000              4/1/2015
      John           3000              4/1/2015
      John          -3000              4/1/2015
      John           2000              5/1/2015
      John          -2000              5/1/2015
      John           2000              5/1/2015
      Tom            1000              1/1/2015
      Tom           -1000              1/1/2015
      Tom            1000              1/1/2015
      Tom            5000              2/1/2015
      Tom           -2000              3/1/2015
      Tom            2000              3/1/2015
      Tom           -2000              3/1/2015
      Tom            2000              3/1/2015
      Tom            2000              3/1/2015
      Tom           -3000              4/1/2015
      Tom            3000              4/1/2015
      Tom           -3000              4/1/2015
                                               ', header=T)
  </code></pre>
  
  <p>I want to get rid of all the Sales that are equal in amount and opposite in sign(+,-)and only show the remaining net Sales(preferably at the earliest date, but it does not matter either way). My desired data frame looks like this</p>
  
  <pre><code>CustomerName    Sales   TradeDate
  John            1000    1/1/2015
  John            5000    2/1/2015
  John            2000    3/3/2015
  John            2000    3/6/2015
  John           -3000    4/1/2015
  John            2000    5/1/2015
  Tom             1000    1/1/2015
  Tom             5000    2/1/2015
  Tom             2000    3/1/2015
  Tom            -3000    4/1/2015
  </code></pre>
  
  <p>I chose two 2000s(in John's case in the month of March) from 3/3/2015 and 3/6/2015. But I am also okay with an output that gave me two 2000s at 3/2/2015 or 3/5/2015. Your help is much appreciated!</p>