[Bug] Elements-23.2.2 with CT discount cannot send full balance less fee

[Impa10r]

When sendtoaddress is called to send the whole liquid balance less margin for fee (75-200 sats) CT discount is not applied and results in:

error code: -6
error message:
Insufficient funds

ElementsProject/peerswap#304 (comment)

We understand there is an option subtractfeefromamount, but prefer not to use it.

[apoelstra]

I'm skeptical that just changing the CT discount logic would make this work (though if you are providing a large margin for fee maybe it will). The fee estimation logic inherently overestimates during calculations and tends to fail near maximum balance.

Upstream added the sendall RPC bitcoin/bitcoin#24118 to cover this usecase, which replaces subtractfeefromamount which was full of weird behavior.