diff --git a/brmbar3/brmbar-cli.py b/brmbar3/brmbar-cli.py
index ca99a73..28fbe87 100755
--- a/brmbar3/brmbar-cli.py
+++ b/brmbar3/brmbar-cli.py
@@ -31,6 +31,9 @@ Usage: brmbar-cli.py COMMAND ARGS...
 		screen of the GUI.
 	adduser USER
 		Add user (debt) account with given username.
+	undo TRANSID
+		Commit a transaction that reverses all splits of a transaction with
+		a given id (to find out that id: select * from transaction_cashsums;)
 
 3. Inventorization
 
@@ -152,6 +155,10 @@ elif sys.argv[1] == "adduser":
     acct.add_barcode(sys.argv[2]) # will commit
     print("{}: id {}".format(acct.name, acct.id));
 
+elif sys.argv[1] == "undo":
+    newtid = shop.undo(int(sys.argv[2]))
+    print("Transaction %d undone by reverse transaction %d" % (int(sys.argv[2]), newtid))
+
 elif sys.argv[1] == "inventory":
     if (len(sys.argv) % 2 != 0 or len(sys.argv) < 4):
         print ("Invalid number of parameters, count your parameters.")
diff --git a/brmbar3/brmbar/Shop.py b/brmbar3/brmbar/Shop.py
index 55c26ba..40b7872 100644
--- a/brmbar3/brmbar/Shop.py
+++ b/brmbar3/brmbar/Shop.py
@@ -177,6 +177,7 @@ class Shop:
             item.credit(transaction, 0, "Inventory fix - amount was correct")
             self.db.commit()
             return False
+
     def fix_cash(self, amount):
         amount_in_reality = amount
         amount_in_system = self.cash.balance()
@@ -196,6 +197,7 @@ class Shop:
             return True
         else:
             return False
+
     def consolidate(self):
         transaction = self._transaction(description = "BrmBar inventory consolidation")
 
@@ -210,3 +212,16 @@ class Shop:
             self.deficit.credit(transaction, deficit_balance, "Deficit balance removed from profit.")
             self.profits.credit(transaction, deficit_balance, "Deficit balance removed from profit.")
         self.db.commit()
+
+    def undo(self, oldtid):
+        description = self.db.execute_and_fetch("SELECT description FROM transactions WHERE id = %s", [oldtid])[0]
+        description = 'undo %d (%s)' % (oldtid, description)
+
+        transaction = self._transaction(description=description)
+        for split in self.db.execute_and_fetchall("SELECT id, side, account, amount, memo FROM transaction_splits WHERE transaction = %s", [oldtid]):
+            splitid, side, account, amount, memo = split
+            memo = 'undo %d (%s)' % (splitid, memo)
+            amount = -amount
+            self.db.execute("INSERT INTO transaction_splits (transaction, side, account, amount, memo) VALUES (%s, %s, %s, %s, %s)", [transaction, side, account, amount, memo])
+        self.db.commit()
+        return transaction