How to send 0.0000004 BTC?

[stwenhao]

but I was just using Electrum as low fee testing wallet, not Bitcoin Core

1. Electrum also has a terminal, and from command line, similar things should be possible.
2. You can export the private key from Electrum to Bitcoin Core, and complete it, even without downloading the blockchain. The only missing part in my example is the private key. If you verify, that everything is correct, then you can safely complete it, also because the risk of losing coins is quite low, because of low amounts.

Edit:

Has anyone succeeded in doing this?

Yes, see: https://mempool.space/tx/49b046366f78ce63014646ba86fe2b2959a4442d6df8c0df603c3821493d7f9d

[LoyceV]

Has anyone succeeded in doing this?

Yes, see: https://mempool.space/tx/49b046366f78ce63014646ba86fe2b2959a4442d6df8c0df603c3821493d7f9d

I assume you used Mara Slipstream, right? The current minimum fee is only 2 sat/vbyte, that's not bad at all.

2. You can export the private key from Electrum to Bitcoin Core

I thought about it, but decided it's not worth the effort.

[stwenhao]

I assume you used Mara Slipstream, right?

Yes. And I also sent some more satoshis as a change, so you can sweep them from bc1qt039dyk3u6x9x3t52nv0dw7evwg2ja7w05gx3v.

The current minimum fee is only 2 sat/vbyte, that's not bad at all.

It is dynamically adjusted. Fortunately, as we can see, spending 40 satoshis is standard, only sending it is not. Which means, that if someone would spam the chain, then collecting it should be standard (I thought it would also lead to non-standard transaction, but it is flying in mempools, so it is probably standard).

I thought about it, but decided it's not worth the effort.

It is not yet worth it, but it may be in the future. And it is good to have an alternative, if on-chain fees will rise, because of next halvings, and if there will be some dust here and there.