Is there a way to undo transactions with a “too low” fee? Code Answer

Suppose we reach a state where only transactions that pay a fee are accepted by miners. Further supposed that someone created a transaction using an old/odd client that allowed them not to send any fees.

In this case, what would happen to the transaction? Could it be undone? Would the coins be lost forever?

Answer

I have successfully used the methodology described here to delete a transaction which didn’t go anywhere because of a too low fee. In short, the process is as follows:

• Configure Linux machine with db4.8-util and bitcointools. Ubuntu works fine.

• Run following command on your data folder:

  ./dbdump.py --datadir ~/.bitcoin --wallet-tx > tx.txt
  

• Open tx.txt using text editor. Find transaction in question. Should look like:

  ==WalletTransaction== 12a75d48a002ab920 ...
  block: 00000000000000000000000000000000 ...
  2 tx in, 1 out
  TxIn: value: 0.007640 prev(c09e92cb94ff1fb6d4 ...
  TxIn: value: 0.000050 prev(7dcda75e6ad8b5adc7 ...
  TxOut: value: 0.007690 pubkey:  Script: DUP HA...
  

• Reverse the 12a75d48a002ab920... value, get something like 92ab02a0485da712. Don’t need to reverse the whole line, just a few hex values from beginning. You will need to search for this in the database dump.

• Dump the database using following command:

  db4.8_dump ~/.bitcoin/wallet.dat > dumpfile
  

• Using text editor open the dumpfile and search for that 92ab02a0485da712 line you found earlier. There will be two lines – key and value, you need to delete both. Save the file.

• Re-import the database using the following command:

  db4.8_load ~/.bitcoin/wallet2.dat < dumpfile
  

If all went fine, the wallet2.dat will have a database without transaction in question.

All of this, of course, only works if transaction was never accepted by the network, which is likely the case if you already waited a day, and nothing happens. There is no way to undo transaction which was already accepted by the network. I tried this technology twice and both times worked fine.