code: public void logPurchase(BigDecimal purchaseAmount, Currency currency, Bundle parameters) {
  
          if (purchaseAmount == null) {
              notifyDeveloperError("purchaseAmount cannot be null");
              return;
          } else if (currency == null) {
              notifyDeveloperError("currency cannot be null");
              return;
          }
  
          if (parameters == null) {
              parameters = new Bundle();
          }
          parameters.putString(AppEventsConstants.EVENT_PARAM_CURRENCY, currency.getCurrencyCode());
  
          logEvent(AppEventsConstants.EVENT_NAME_PURCHASED, purchaseAmount.doubleValue(), parameters);
          eagerFlush();
      }
docs: Logs a purchase event with Facebook, in the specified amount and with the specified currency.  Additional
  detail about the purchase can be passed in through the parameters bundle.
  
  @param purchaseAmount Amount of purchase, in the currency specified by the 'currency' parameter. This value
  will be rounded to the thousandths place (e.g., 12.34567 becomes 12.346).
  @param currency       Currency used to specify the amount.
  @param parameters     Arbitrary additional information for describing this event.  Should have no more than
  10 entries, and keys should be mostly consistent from one purchase event to the next.