I tried the latest master branch containing that PR but it can't seem to sign the lowest possible 0.1sat/vB transaction at first.
But after manually adding 1sat to the absolute fee, it let me sign the transaction.
Seem to be a minor issue with fee rate calculation since it's too small.
Do you have more details to reproduce the rounding error? What was the vbyte size of the transaction?
You set the feerate to 0.1 s/b, and what absolute fee did the GUI calculate?
(EDIT: I think I found it. should be fixed by
https://github.com/spesmilo/electrum/commit/392400295e733e1dd3f0cb14189466e8a76890ef)
In that case
the old method should still work. You can set
wallet.relayfee = (lambda: 0)
in the Qt console, and then the GUI lets you do whatever you want. Create even zero-fee txs.
But towards the end of that GitHub thread they say that this method doesn't work anymore.
It works. That's why the issue got closed, and I tested it before posting that here in any case.
But what I noticed was that the equivalent amount in USD was as if I had set the fee to 1 sat/vByte. The wallet didn't lower it regardless if I set it to 0.7, 0.5, 0.3 sats, etc. There was also a notification next to the total fee that said "xxx amount of sats will be added to the transaction." It always added enough to make the fee 1 sat/vByte.
If you have "output value rounding" enabled, that can increase the fee by up to 100 sats.
Also, dust-valued change outputs are not created but converted to fees.
electrum.labrie.ca
Confirmed, I was able to make a 0.3 sat/vB tx using this server.
I don't know how you got it to work because it didn't work for me. After I switched to the server, I saw the usual notification that the server won't broadcast my transaction when I set the transaction fee below 1 sat.
What version of the client were you running? Let me repeat it again, it will not work on any released version. You need the linked PR - and - to be connected to a low-fee server. You need both.
OR you can just run that one-liner in the console:
wallet.relayfee = (lambda: 0)
That in itself is sufficient.