How do I get the change back into the original wallet?

[wilonx]

Hi everyone, I'm developing an integration with the bitcoinlib library in Python and I'm having trouble managing the change address.

Currently, when making a transaction (especially in Segwit/Bech32 wallets), the library automatically generates a new change address (following the derivation path m/84'/0'/0'/1/x). However, for organizational purposes and simple balance monitoring, I need all the remaining balance from the transaction (the change) to return to the exact same originating address of the transaction, instead of creating a new address.

w = wallet_create_or_open(tx_request.get("fromWalletAddress", {}), keys=MNEMONIC, network='bitcoin', witness_type='segwit')
outputs = []
out_payment = Output(value=amount_sats, address=destino, network="bitcoin")
outputs.append(out_payment)

if memo_hex:
script_bytes = bytes.fromhex(memo_hex)
#print(script_bytes)

# Ref: Documentação bitcoinlib - script_type='nulldata' para OP_RETURN
out_memo = Output(value=0, lock_script=script_bytes, script_type='nulldata', network="bitcoin")
outputs.append(out_memo)

t = w.transaction_create(
output_arr=outputs,
fee='normal',
min_confirms=1
)

In one screenshot, the output is:

bc1q4y8kgvhvky465vv5y3yxlu8cjuuqzw3erzgfhz 0.00007274 BTC

to

3CAQAw7m95axbY761Xq8d9DADhjNaX9b8o 0.00003181 BTC

bc1q38zsl6auxyg6ckc3r94ry289f0m7xq8n9nz08e 0.00003869 BTC

This wallet nz08e is mine, but I want the change to go back to the wallet gfhz… the other screenshot shows how I would like it…

In this example, the entire value of 0eeee comes out, a little goes to my wallet (gfhz), but the change goes to 0eeee again:

from

bc1prka6nc0dln70wv9znw49huqssdkdvsfsfl6pfqnyq7lsjug5yjhsl0eeee
‎0.00006086 BTC
bc1prka6nc0dln70wv9znw49huqssdkdvsfsfl6pfqnyq7lsjug5yjhsl0eeee
‎0.00826732 BTC

to

bc1q4y8kgvhvky465vv5y3yxlu8cjuuqzw3erzgfhz
‎0.00091594 BTC

bc1prka6nc0dln70wv9znw49huqssdkdvsfsfl6pfqnyq7lsjug5yjhsl0eeee
‎0.00740960 BTC

[mccwdev]

The transaction fee is the sum of all inputs minus the sum of all inputs, so if you create enough output value no extra change address is needed.

For example:

pkwif = 'BC19UtECk2r9PVQYhYH3by5bMhUpzg97pFSNiM1fH2Xb7t87JAem4WHh8KL824sAdGqsxMsGAoq7C5KVY8htddF3Lb5LW8FnxpMzCTwBYKv8AaR5'

w = wallet_create_or_open('bcl_test_wallet', pkwif)
w.utxos_update()
w.info()

outputs = []
outputs.append(Output(value=0, lock_script='6a0160', script_type='nulldata', network="bitcoinlib_test"))
outputs.append(Output(30000000, 'blt1qqpap3e5m8udm2kp7gatchacqu66x54ktqqr2ut', network="bitcoinlib_test"))
outputs.append(Output(69999000, 'blt1qu345x9thyydhj4dfjxwnz2f9gr5ua4um3w50lw', network="bitcoinlib_test"))

t = w.transaction_create(outputs, fee=1000)
t.info()

If needed you can specify inputs with input_arr or input_key_id

[wilonx]

You didn't understand what I meant, let's go:

You have 6468 sats in your wallet.

You want to send 1000 to a wallet, so you have to pass the change (the remaining amount) that will be sent to another wallet of yours, because this balance is yours...

Therefore, this balance that will be sent as change is 5289, already considering the 179 fee, example below:

Now I ask you, my wallet is bc1q4y8kgvhvky465vv5y3yxlu8cjuuqzw3erzgfhz, from where the 1000 and the remainder (5289) are coming, but bitcoinlib, instead of sending the 5289 back to bc1q4y8kgvhvky465vv5y3yxlu8cjuuqzw3erzgfhz, it creates a secondary wallet and sends the change to it. bc1qh497lwnfxa4jc3mm6vgts5yuvfexeq57pfvtqk

So my question is, how do I send the change to bc1q4y8kgvhvky465vv5y3yxlu8cjuuqzw3erzgfhz?

        w = wallet_create_or_open(wallet, keys=private_key_wif, network='bitcoin', witness_type='segwit')
        w.utxos_update()

        outputs = []

        out_payment = Output(value=1000, address="16je3T8z94L24mrHD1N4ft9L7p2bHXPFg7", network="bitcoin")
        outputs.append(out_payment)
        t = w.transaction_create(
            output_arr=outputs,
            fee='high',
            min_confirms=1
        )
        t.info()

t.info() show this:

Inputs
- bc1q4y8kgvhvky465vv5y3yxlu8cjuuqzw3erzgfhz 0.00006468 BTC 4807ca156f64c32dee6a12941929ed831c22e438692803a2effabb0726094389 0
  segwit sig_pubkey; sigs: 0 (1-of-1) not validated
Outputs
- 16je3T8z94L24mrHD1N4ft9L7p2bHXPFg7 0.00001000 BTC p2pkh U
- bc1qh497lwnfxa4jc3mm6vgts5yuvfexeq57pfvtqk 0.00005289 BTC p2wpkh UC
fee: 179

[mccwdev]

I'm not sure if I can follow you, but my example above seems to solve your problem.

You have to set a fee manually, because the fee='high' option does not give a fixed result, it depends on the current network fees.