"private" agreement between two c-lightning nodes on channel closure fee When i was at LNConf, one of the devs of c-lightning (IIRC) told me that there is a custom, manual way to agree between two nodes on channel closing fees.

Can anyone elaborate how this is possible? I am operator of two cln nodes, i would like to close channel between these nodes with minimum possible fee, because i am not in hurry. 

Are there some side-effects of doing so?
Just use lightning-cli close id the implementation will first try to do a mutual close. Both lightning nodes will use the fee estimator from Bitcoind to suggest fees. If they don't agree there is a communication protocol defined in the BOLTs that they will eventually find a local agreement between themselves. This protocol is described under the section closing negotiation in BOLT 02 c.f.: https://github.com/lightningnetwork/lightning-rfc/blob/master/02-peer-protocol.md#closing-negotiation-closing_signed

Obviously both nodes need to be online and peered before you call the close command otherwise the node will do an expensive force close. Also it makes sense to wait until fees in the mempool are low if you can do that.