Index: ps/trunk/binaries/data/mods/public/gui/session/menu.js
===================================================================
--- ps/trunk/binaries/data/mods/public/gui/session/menu.js
+++ ps/trunk/binaries/data/mods/public/gui/session/menu.js
@@ -29,6 +29,7 @@
 const g_IdleTraderTextColor = "orange";
 
 /**
+ * The barter constants should match with the simulation
  * Quantity of goods to sell per click.
  */
 const g_BarterResourceSellQuantity = 100;
Index: ps/trunk/binaries/data/mods/public/simulation/components/Barter.js
===================================================================
--- ps/trunk/binaries/data/mods/public/simulation/components/Barter.js
+++ ps/trunk/binaries/data/mods/public/simulation/components/Barter.js
@@ -87,6 +87,8 @@
 		warn("ExchangeResources: player has no markets");
 		return;
 	}
+	if (amount != 100 && amount != 500)
+		return;
 
 	var cmpPlayer = Engine.QueryInterface(playerEntity, IID_Player);
 	var prices = this.GetPrices(playerEntity);
@@ -96,7 +98,6 @@
 	{
 		var amountToAdd = Math.round(prices["sell"][resourceToSell] / prices["buy"][resourceToBuy] * amount);
 		cmpPlayer.AddResource(resourceToBuy, amountToAdd);
-		var numberOfDeals = Math.round(amount / 100);
 
 		// Display chat message to observers
 		var cmpGUIInterface = Engine.QueryInterface(SYSTEM_ENTITY, IID_GuiInterface);
@@ -117,11 +118,12 @@
 			cmpStatisticsTracker.IncreaseResourcesBoughtCounter(resourceToBuy, amountToAdd);
 		}
 
+		let difference = this.DIFFERENCE_PER_DEAL * amount / 100;
 		// Increase price difference for both exchange resources.
 		// Overall price difference (dynamic +/- constant) can't exceed +-99%.
-		this.priceDifferences[resourceToSell] -= this.DIFFERENCE_PER_DEAL * numberOfDeals;
+		this.priceDifferences[resourceToSell] -= difference;
 		this.priceDifferences[resourceToSell] = Math.min(99 - this.CONSTANT_DIFFERENCE, Math.max(this.CONSTANT_DIFFERENCE - 99, this.priceDifferences[resourceToSell]));
-		this.priceDifferences[resourceToBuy] += this.DIFFERENCE_PER_DEAL * numberOfDeals;
+		this.priceDifferences[resourceToBuy] += difference;
 		this.priceDifferences[resourceToBuy] = Math.min(99 - this.CONSTANT_DIFFERENCE, Math.max(this.CONSTANT_DIFFERENCE - 99, this.priceDifferences[resourceToBuy]));
 	}
 
Index: ps/trunk/binaries/data/mods/public/simulation/components/tests/test_Barter.js
===================================================================
--- ps/trunk/binaries/data/mods/public/simulation/components/tests/test_Barter.js
+++ ps/trunk/binaries/data/mods/public/simulation/components/tests/test_Barter.js
@@ -120,14 +120,24 @@
 TS_ASSERT_EQUALS(sold, 100);
 TS_ASSERT_EQUALS(bought, Math.round(100 * (100 - cmpBarter.CONSTANT_DIFFERENCE + 0) / (100 + cmpBarter.CONSTANT_DIFFERENCE + 0)));
 
-// With an amount which is not a multiple of 100, price differences are not updated. That sounds wrong.
+// Amount which is not 100 or 500 is invalid.
 cmpBarter.priceDifferences = { "wood": 0, "stone": 0, "metal": 0 };
 cmpBarter.ExchangeResources(11, "wood", "stone", 40);
-TS_ASSERT_EQUALS(cmpBarter.restoreTimer, 7);
-TS_ASSERT(timerActivated);
+bought = 0;
+sold = 0;
+timerActivated = false;
+TS_ASSERT(!timerActivated);
+TS_ASSERT_UNEVAL_EQUALS(cmpBarter.priceDifferences, { "wood": 0, "stone": 0, "metal": 0 });
+TS_ASSERT_EQUALS(sold, 0);
+TS_ASSERT_EQUALS(bought, 0);
+
+// Amount which is NaN is invalid.
+cmpBarter.priceDifferences = { "wood": 0, "stone": 0, "metal": 0 };
+cmpBarter.ExchangeResources(11, "wood", "stone", NaN);
+TS_ASSERT(!timerActivated);
 TS_ASSERT_UNEVAL_EQUALS(cmpBarter.priceDifferences, { "wood": 0, "stone": 0, "metal": 0 });
-TS_ASSERT_EQUALS(sold, 40);
-TS_ASSERT_EQUALS(bought, Math.round(40 * (100 - cmpBarter.CONSTANT_DIFFERENCE + 0) / (100 + cmpBarter.CONSTANT_DIFFERENCE + 0)));
+TS_ASSERT_EQUALS(sold, 0);
+TS_ASSERT_EQUALS(bought, 0);
 
 cmpBarter.priceDifferences = { "wood": 0, "stone": 99 - cmpBarter.CONSTANT_DIFFERENCE, "metal": 0 };
 cmpBarter.ExchangeResources(11, "wood", "stone", 100);