### 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_
{ "sendall", 4, "locktime"},
    { "sendall", 4, "lock_unspents"},
    { "sendall", 4, "psbt"},
    { "sendall", 4, "send_max"},
    { "sendall", 4, "minconf"},
    { "sendall", 4, "maxconf"},
    { "sendall", 4, "conf_target"},
    { "sendall", 4, "replaceable"},
    { "sendall", 4, "solving_data"},
    { "simulaterawtransaction", 0, "rawtxs" },
    { "simulaterawtransaction", 1, "options" },
    { "simulaterawtransaction", 1, "include_watchonly"},
    { "importprivkey", 2, "rescan" },
    { "importaddress", 2, "rescan" },
    { "importaddress", 3, "p2sh" },
    { "importpubkey", 2, "rescan" },
    { "importmempool", 1, "options" },
    { "importmempool", 1, "apply_fee_delta_priority" },
    { "importmempool", 1, "use_current_time" },
    { "importmempool", 1, "apply_unbroadcast_set" },
    { "importmulti", 0, "requests" },
    { "importmulti", 1, "options" },
    { "importmulti", 1, "rescan" },
    { "importdescriptors", 0, "requests" },
    { "listdescriptors", 0, "private" },
    { "verifychain", 0, "checklevel" },
    { "verifychain", 1, "nblocks" },
    { "getblockstats", 0, "hash_or_height" },
    { "getblockstats", 1, "stats" },
    { "pruneblockchain", 0, "height" },