swap: Leave some fees to send to the swap service

[cdecker]

swap: Leave some fees to send to the swap service

[roeierez]

Thanks @cdecker for the anlysis. We have a function in the sdk onchain_payment_limits which should return the max amount the user can swap out. we do that by composing the shortest path to the swapper and reverse comuting the fees so the amount the user sees is the channels balance minus the fees. Then we use send_pay to send using this specific route.
If the user ignores this value and attempts to send all the balance then might fall into this.
Is it connected to any supsport case where I can return to the user and ask for some info? Perhaps relai?

[cdecker]

I see, yes, in that case the behavior is correct, since the SDK accounts for fees. I misinterpreted the amounts matching as the situation at the recipient (sans fees) and not at the sender (with fees). So this is all good, I just got confused.