Transferring all funds from a custodial service over Lightning Network

Question

This is the third time (meaning three different services) something like this has happened to me: A custodial service holds a small amount of my bitcoin. When I want to withdraw all funds to my own Lightning node, the transaction fails with an error message that looks a little like this:

You don't have enough balance to pay the route fees, try a lower amount.

Usually what follows is that I manually lower the amount to send and a dust amount is left in my account which I never withdraw.

What's the problem here? Is it not possible to implement a "send all" function that would deduct the fees from the total amount?

Answer 1

I have observed the same behavior and I think a "send all" function should be possible to build but is just not implemented. It is a little bit tricky as the actual routing fees depend on the path / route and might change a bit with every attempt and are not known before the payment settles. But of course a custodial service could work with a fee budget that slightly overpays the fees in order to introduce a better user experience.