diff --git a/lib/routes.js b/lib/routes.js
index 4621fd29..0ac54dbe 100644
--- a/lib/routes.js
+++ b/lib/routes.js
@@ -19,19 +19,21 @@ Error.prototype.toJSON = function () {
 
 var poll = function(req, res) {
   var rateRec = _trader.rate();
-  var satoshiBalanceRec = _trader.balance;
+  var balanceRec = _trader.balance;
+  var fingerprint = req.connection.getPeerCertificate().fingerprint;
 
-  // `rateRec` and `satoshiBalanceRec` are both objects, so there's no danger
+  // `rateRec` and `balanceRec` are both objects, so there's no danger
   // of misinterpreting rate or balance === 0 as 'Server initializing'.
-  if (!rateRec || !satoshiBalanceRec) {
+  if (!rateRec || !balanceRec) {
     return res.json({err: 'Server initializing'});
   }
 
-  if (Date.now() - rateRec.timestamp > STALE_TICKER) {
+  var now = Date.now();
+  if (now - rateRec.timestamp > STALE_TICKER) {
     return res.json({err: 'Stale ticker'});
   }
 
-  if (Date.now() - rateRec.timestamp > STALE_BALANCE) {
+  if (now - balanceRec.timestamp > STALE_BALANCE) {
     return res.json({err: 'Stale balance'});
   }
 
@@ -40,16 +42,16 @@ var poll = function(req, res) {
   res.json({
     err: null,
     rate: rate * _trader.config.exchanges.settings.commission,
-    fiat: _trader.fiatBalance(0, 0),
+    fiat: _trader.fiatBalance(fingerprint),
     locale: _trader.config.brain.locale,
     txLimit: parseInt(_trader.config.exchanges.settings.compliance.maximum.limit, 10)
   });
 };
 
 var trade = function (req, res) {
-  _trader.trade(req.body.fiat, req.body.satoshis, req.body.currency, function(err) {
-    res.json({err: err});
-  });
+  var fingerprint = req.connection.getPeerCertificate().fingerprint;
+  _trader.trade(req.body, fingerprint);
+  res.json({err: null});
 };
 
 var send = function(req, res) {
diff --git a/lib/trader.js b/lib/trader.js
index 4be3b5fd..0dc269af 100644
--- a/lib/trader.js
+++ b/lib/trader.js
@@ -6,6 +6,9 @@ var winston = require('winston');
 
 var SATOSHI_FACTOR = Math.pow(10, 8);
 
+// TODO: Define this somewhere more global
+var SESSION_TIMEOUT = 60 * 60 * 1000;   // an hour
+
 var Trader = module.exports = function (db) {
   if (!db) {
     throw new Error('`db` is required');
@@ -18,6 +21,7 @@ var Trader = module.exports = function (db) {
   });
 
   this._tradeQueue = [];
+  this._sessionInfo = {};
 };
 
 Trader.prototype._findExchange = function (name) {
@@ -79,12 +83,13 @@ Trader.prototype._consolidateTrades = function () {
   return tradeRec;
 };
 
-Trader.prototype._purchase = function (trade) {
+Trader.prototype._purchase = function (trade, cb) {
   var self = this;
   var rate = self.rate(trade.currency);
   self.tradeExchange.purchase(trade.satoshis, rate.rate, function (err) {
-    // TODO: don't ignore purchase errors
+    if (err) return cb(err);
     self.pollBalance();
+    cb();
   });
 };
 
@@ -114,65 +119,60 @@ Trader.prototype.configure = function (config) {
   this.pollRate();
 };
 
-/**
- * return fiat balance
- *
- * in input to this function, balance has the following parameters...
- *
- * balance.transferBalance - in satoshis
- * balance.tradeBalance    - in USD
- *
- * Have added conversion here, but this really needs to be thought through, lamassu-bitstamp should perhaps
- * return balance in satoshis
- */
-Trader.prototype.fiatBalance = function (transferSatoshis, tradeFiat) {
-  var rate = this.rate(this.config.exchanges.settings.currency).rate;
+// IMPORTANT: This function returns the estimated minimum available balance
+// in fiat as of the start of the current user session on the device. User 
+// session starts when a user presses then Start button and ends when we
+// send the bitcoins.
+Trader.prototype.fiatBalance = function (deviceFingerprint) {
+  var rawRate = this.rate(this.config.exchanges.settings.currency).rate;
   var balance = this.balance;
   var commission = this.config.exchanges.settings.commission;
 
-  if (!rate || !balance) {
+  if (!rawRate || !balance) {
     return 0;
   }
 
   // The rate is actually our commission times real rate.
-  rate = commission * rate;
+  var rate = commission * rawRate;
 
   // `lowBalanceMargin` is our safety net. It's a number > 1, and we divide
   // all our balances by it to provide a safety margin.
   var lowBalanceMargin = this.config.exchanges.settings.lowBalanceMargin;
 
   // `balance.transferBalance` is the balance of our transfer account (the one
-  // we use to send Bitcoins to clients). `transferSatoshis` is the number
-  // of satoshis we're expected to send for this transaction. By subtracting
-  // them, we get `adjustedTransferBalance`, amount of satoshis we'll have
-  // after the transaction.
-  var adjustedTransferBalance = balance.transferBalance - transferSatoshis;
+  // we use to send Bitcoins to clients).
+  var transferBalance = balance.transferBalance;
   
-  // Since `adjustedTransferBalance` is in Satoshis, we need to turn it into
-  // Bitcoins and then fiat to learn how much fiat currency we can exchange.
+  // Since `transferBalance` is in satoshis, we need to turn it into
+  // bitcoins and then fiat to learn how much fiat currency we can exchange.
   //
   // Unit validity proof: [ $ ] = [ (B * 10^8) / 10^8 * $/B ]
   //                      [ $ ] = [ B * $/B ]
   //                      [ $ ] = [ $ ]
-  var fiatTransferBalance = ((adjustedTransferBalance / SATOSHI_FACTOR) * rate) / lowBalanceMargin;
+  var fiatTransferBalance = ((transferBalance / SATOSHI_FACTOR) * rate) / lowBalanceMargin;
 
   // If this server is also configured to trade received fiat for Bitcoins,
   // we also need to calculate if we have enough funds on our trade exchange.
   if (balance.tradeBalance === null) return fiatTransferBalance;
   var tradeBalance = balance.tradeBalance;
 
-  // We need to secure `tradeFiat` (amount of fiat in this transaction) and
-  // enough fiat to cover our trading queue (trades aren't executed immediately).
-  var adjustedFiat = tradeFiat + this._tradeQueueFiatBalance(rate);
-
-  // So we subtract `adjustedFiat` from `tradeBalance` and again, apply
-  // `lowBalanceMargin`.
-  var fiatTradeBalance = (tradeBalance - adjustedFiat) / lowBalanceMargin;
+  // We're reporting balance as of the start of the user session.
+  var sessionInfo = this._sessionInfo[deviceFingerprint];
+  var sessionBalance = sessionInfo ? sessionInfo.tradeBalance : tradeBalance;
+  var fiatTradeBalance = sessionBalance / lowBalanceMargin;
 
   // And we return the smallest number.
   return Math.min(fiatTransferBalance, fiatTradeBalance);
 };
 
+Trader.prototype._clearSession = function (deviceFingerprint) {
+  var sessionInfo = this._sessionInfo[deviceFingerprint];
+  if (sessionInfo) {
+    clearTimeout(sessionInfo.reaper);
+    delete this._sessionInfo[deviceFingerprint];
+  }
+};
+
 Trader.prototype.sendBitcoins = function (deviceFingerprint, tx, cb) {
   var self = this;
 
@@ -182,6 +182,7 @@ Trader.prototype.sendBitcoins = function (deviceFingerprint, tx, cb) {
     }
 
     if (isNew) {
+      this._clearSession(deviceFingerprint);
       return self.transferExchange.sendBitcoins(
         tx.toAddress,
         tx.satoshis,
@@ -205,9 +206,21 @@ Trader.prototype.sendBitcoins = function (deviceFingerprint, tx, cb) {
   });
 };
 
-Trader.prototype.trade = function (fiat, satoshis, currency, callback) {
-  this._tradeQueue.push({fiat: fiat, satoshis: satoshis, currency: currency});
-  callback(null);
+Trader.prototype.trade = function (rec, deviceFingerprint) {
+  // This is where we record starting trade balance at the beginning
+  // of the user session
+  var sessionInfo = this._sessionInfo[deviceFingerprint];
+  var self = this;
+  if (!sessionInfo) {
+    this._sessionInfo[deviceFingerprint] = {
+      tradeBalance: this.balance.tradeBalance,
+      timestamp: Date.now(),
+      reaper: setTimeout(function () {
+        delete self._sessionInfo[deviceFingerprint];
+      }, SESSION_TIMEOUT)
+    };
+  } 
+  this._tradeQueue.push({fiat: rec.fiat, satoshis: rec.satoshis, currency: rec.currency});
 };
 
 Trader.prototype.executeTrades = function () {
@@ -231,7 +244,10 @@ Trader.prototype.executeTrades = function () {
   }
 
   this.logger.info('making a trade: %d', trade.satoshis / Math.pow(10, 8));
-  this._purchase(trade);
+  var self = this;
+  this._purchase(trade, function (err) {
+    if (err) self.logger.error(err);
+  });
 };
 
 Trader.prototype.startPolling = function () {