- /// Note that if the preimage is not known, you must call ChannelManager::fail_htlc_backwards
- /// to free up resources for this HTLC.
+ /// Note that if the preimage is not known or the amount paid is incorrect, you must call
+ /// ChannelManager::fail_htlc_backwards with PaymentFailReason::PreimageUnknown or
+ /// PaymentFailReason::AmountMismatch, respectively, to free up resources for this HTLC.
+ /// The amount paid should be considered 'incorrect' when it is less than or more than twice
+ /// the amount expected.