<p>I sent an email requesting an explanation from Muun and their support service were so kind and gave me a very informative response. Here is their email:</p>
<blockquote>
<p>The reason for your transaction paying a higher fee than what you chose is that your transaction had a very tiny change output. When this happens, Muun and many other bitcoin wallets use the extra money to increase the network fee paid to miners.
To fully understand this, you need to be familiar with bitcoin’s UTXO model. I’m happy to share some information on the topic.
Unlike the traditional financial system, bitcoin does not use accounts and balances. Instead, users own pieces of bitcoin called UTXOs.
Every bitcoin transaction is composed of inputs and outputs. Inputs consume an existing UTXO, while outputs create a new UTXO. When you make a transaction, your wallet selects at least one UTXO to spend and creates new outputs. One output will go to the receiver’s address, and the other will return to your wallet to a different address. This second output is called the change output.
If the change output happens to be too small, it may cost more in fees to spend it than it is worth, or, even more, it may not be viable. When this happens, your wallet will use the tiny change output to boost your transaction fee.</p>
</blockquote>

CC BY-SA 4.0
2024-12-24T18:05:34.207Z
2024-12-24T18:05:34.207Z
a.sa.5969
<p>Looking at the transaction, it seems that the wallet actually has 10,646 sats. However, in the past, Muun has sent more to users (via swaps) than they are supposed to receive due to minimum amount requirements, and hidden the overage on the client side. It is possible they are still doing that and that is why you only saw 10,500 sats.</p>
<p>When you went to send 10,000 sats, the resulting transaction has a size of 157 vbytes, and with a feerate of 1 sat/vb, the fee should have been 157 sats. It seems that their size estimation is significantly overestimating if they estimated the fee to be 220 sats with a feerate of 1 sat/vb.</p>
<p>With a fee of 157 sats, the change output would have been 489 sats. Although for all segwit versions, this is greater than the dust threshold of 203 sats, it is less than the dust threshold for legacy addresses of 546 sats. A quick perusal of Muun's code suggests that they still check against the 546 sat threshold. Since they thought that the change output would be dust, and therefore make it harder for the transaction to propagate, they chose to discard that remainder into the transaction fee rather than create a change output.</p>

CC BY-SA 4.0
2024-12-24T06:05:54.227Z
2024-12-24T06:05:54.227Z
Ava Chow
<p>I am using <code>Muun</code> Wallet. I had a total of 10,500 Sats in my wallet.</p>
<p>I sent 10,000 Sats to a BTC wallet. I chose the fee rate of 1 sat/vB (total 220).</p>
<p>So, I was expecting that my wallet would have 280 Sats remaining. But, I see that all my assets are gone now. On the other hand, I expected to see a fee of 220 Sats and a rate of 1 sat/vB on &quot;mempool.com&quot;. But, I see that the fee was 646 sats and the rate was 4.11 sat/vB.</p>
<p>Could you please explain to me what happened here and where are the remaining 280 Sats? With this intransparency, can I trust <code>Muun</code> for future larger transactions?!</p>

CC BY-SA 4.0
2024-12-23T22:19:02.990Z
Can you provide the transaction id? Your numbers do not add up. If you had 10,500 sats and sent 10,000 sats, then there is not enough remaining for a fee of 646 sats.
Ava Chow
CC BY-SA 4.0
2024-12-24T04:19:06.297Z
Yes, I know!!!! Muun is wild! Here is the [tx id](https://mempool.space/tx/e7b8aaa7864422a36041523477c732e9b33e3a9833312b3c1d86e12acd782242). I saw that others have raised similar concerns, too: [here](https://stacker.news/items/84104#:~:text=The%20wallet%20says%20I%20paid%20751691%20sats%20but%20the%20on%2Dchain%20explorer%20shows%20a%20different%20amount) and [here](https://www.reddit.com/r/lightningnetwork/comments/mrxl6e/muun_vs_phoenix_vs_other_for_lightning_network/)
a.sa.5969
<wallet><lightning-network><transaction-fees><submarine-swaps><muun>
Why does Muun change the fee without permission?
a.sa.5969
# Why does Muun change the fee without permission?

(Asked by: username_0 on 2024-12-23)

I am using `Muun` Wallet. I had a total of 10,500 Sats in my wallet.

I sent 10,000 Sats to a BTC wallet. I chose the fee rate of 1 sat/vB (total 220).

So, I was expecting that my wallet would have 280 Sats remaining. But, I see that all my assets are gone now. On the other hand, I expected to see a fee of 220 Sats and a rate of 1 sat/vB on "mempool.com". But, I see that the fee was 646 sats and the rate was 4.11 sat/vB.

Could you please explain to me what happened here and where are the remaining 280 Sats? With this intransparency, can I trust `Muun` for future larger transactions?!

**Tags:** wallet, lightning-network, transaction-fees, submarine-swaps, muun

> **Comments:**
> **username_1**: Can you provide the transaction id? Your numbers do not add up. If you had 10,500 sats and sent 10,000 sats, then there is not enough remaining for a fee of 646 sats.  
> **username_0**: Yes, I know!!!! Muun is wild! Here is the [tx id](https://mempool.space/tx/e7b8aaa7864422a36041523477c732e9b33e3a9833312b3c1d86e12acd782242). I saw that others have raised similar concerns, too: [here](https://stacker.news/items/84104#:~:text=The%20wallet%20says%20I%20paid%20751691%20sats%20but%20the%20on%2Dchain%20explorer%20shows%20a%20different%20amount) and [here](https://www.reddit.com/r/lightningnetwork/comments/mrxl6e/muun_vs_phoenix_vs_other_for_lightning_network/)  

---

## 2 answers

### Answer by username_0 on 2024-12-24. Score: 1 | Accepted answer

I sent an email requesting an explanation from Muun and their support service were so kind and gave me a very informative response. Here is their email:

> The reason for your transaction paying a higher fee than what you chose is that your transaction had a very tiny change output. When this happens, Muun and many other bitcoin wallets use the extra money to increase the network fee paid to miners.
> To fully understand this, you need to be familiar with bitcoin’s UTXO model. I’m happy to share some information on the topic.
> Unlike the traditional financial system, bitcoin does not use accounts and balances. Instead, users own pieces of bitcoin called UTXOs.
> Every bitcoin transaction is composed of inputs and outputs. Inputs consume an existing UTXO, while outputs create a new UTXO. When you make a transaction, your wallet selects at least one UTXO to spend and creates new outputs. One output will go to the receiver’s address, and the other will return to your wallet to a different address. This second output is called the change output.
> If the change output happens to be too small, it may cost more in fees to spend it than it is worth, or, even more, it may not be viable. When this happens, your wallet will use the tiny change output to boost your transaction fee.


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### Answer by username_1 on 2024-12-24. Score: 3

Looking at the transaction, it seems that the wallet actually has 10,646 sats. However, in the past, Muun has sent more to users (via swaps) than they are supposed to receive due to minimum amount requirements, and hidden the overage on the client side. It is possible they are still doing that and that is why you only saw 10,500 sats.

When you went to send 10,000 sats, the resulting transaction has a size of 157 vbytes, and with a feerate of 1 sat/vb, the fee should have been 157 sats. It seems that their size estimation is significantly overestimating if they estimated the fee to be 220 sats with a feerate of 1 sat/vb.

With a fee of 157 sats, the change output would have been 489 sats. Although for all segwit versions, this is greater than the dust threshold of 203 sats, it is less than the dust threshold for legacy addresses of 546 sats. A quick perusal of Muun's code suggests that they still check against the 546 sat threshold. Since they thought that the change output would be dust, and therefore make it harder for the transaction to propagate, they chose to discard that remainder into the transaction fee rather than create a change output.


---

