### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current behaviour

See inline discussion at https://github.com/bitcoin/bitcoin/pull/26467#discussion_r1269177446

@whitslack:

> Consider the case where I am paying someone on chain under the stipulation that _they_ will eat the mining fee. (Thus, I specify _their_ address in `subtractfeefrom` when I'm constructing my transaction.) Later, they tell me that they can't wait for confirmation any longer and need an urgent fee bump. If I naïvely specify their output as the `reduce_output`, I may end up paying them _**much more**_ than I intended to if I only have large UTxOs in my wallet.


### Expected behaviour

Not sure.

Improving the docs is one option, but this behaviour seem quite counter intuitive.

My own suggestion:

> Should we disable the adding of new inputs when `reduce_output` is set? If someone really intends to do that, they should probably use `outputs`.

### Steps to reproduce

Haven't tried.

### Relevant log output

_No response_

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

Master after #26467

### Operating system and version

Any

### Machine specifications

_No response_
{ "getbalance", 3, "avoid_reuse" },
    { "getblockfrompeer", 1, "peer_id" },
    { "getblockhash", 0, "height" },
    { "waitforblockheight", 0, "height" },
    { "waitforblockheight", 1, "timeout" },
    { "waitforblock", 1, "timeout" },
    { "waitfornewblock", 0, "timeout" },
    { "listtransactions", 1, "count" },
    { "listtransactions", 2, "skip" },
    { "listtransactions", 3, "include_watchonly" },
    { "walletpassphrase", 1, "timeout" },
    { "getblocktemplate", 0, "template_request" },
    { "listsinceblock", 1, "target_confirmations" },
    { "listsinceblock", 2, "include_watchonly" },
    { "listsinceblock", 3, "include_removed" },
    { "listsinceblock", 4, "include_change" },
    { "sendmany", 1, "amounts" },
    { "sendmany", 2, "minconf" },
    { "sendmany", 4, "subtractfeefrom" },
    { "sendmany", 5 , "replaceable" },
    { "sendmany", 6 , "conf_target" },
    { "sendmany", 8, "fee_rate"},
    { "sendmany", 9, "verbose" },
    { "deriveaddresses", 1, "range" },
    { "scanblocks", 1, "scanobjects" },
    { "scanblocks", 2, "start_height" },
    { "scanblocks", 3, "stop_height" },
    { "scanblocks", 5, "options" },
    { "scantxoutset", 1, "scanobjects" },
    { "addmultisigaddress", 0, "nrequired" },