### 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_
"Only solvable inputs can be used. Watch-only destinations are solvable if the public key and/or output script was imported,\n"
                                              "e.g. with 'importpubkey' or 'importmulti' with the 'pubkeys' or 'desc' field."},
                        {"inputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Use exactly the specified inputs to build the transaction. Specifying inputs is incompatible with the send_max, minconf, and maxconf options.",
                            {
                                {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
                                    {
                                        {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                                        {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                                        {"sequence", RPCArg::Type::NUM, RPCArg::DefaultHint{"depends on the value of the 'replaceable' and 'locktime' arguments"}, "The sequence number"},
                                    },
                                },
                            },
                        },
                        {"locktime", RPCArg::Type::NUM, RPCArg::Default{0}, "Raw locktime. Non-0 value also locktime-activates inputs"},
                        {"lock_unspents", RPCArg::Type::BOOL, RPCArg::Default{false}, "Lock selected unspent outputs"},
                        {"psbt", RPCArg::Type::BOOL,  RPCArg::DefaultHint{"automatic"}, "Always return a PSBT, implies add_to_wallet=false."},
                        {"send_max", RPCArg::Type::BOOL, RPCArg::Default{false}, "When true, only use UTXOs that can pay for their own fees to maximize the output amount. When 'false' (default), no UTXO is left behind. send_max is incompatible with providing specific inputs."},
                        {"minconf", RPCArg::Type::NUM, RPCArg::Default{0}, "Require inputs with at least this many confirmations."},
                        {"maxconf", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "Require inputs with at most this many confirmations."},
                    },
                    FundTxDoc()
                ),
                RPCArgOptions{.oneline_description="options"}
            },
        },
        RPCResult{
            RPCResult::Type::OBJ, "", "",
                {
                    {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
                    {RPCResult::Type::STR_HEX, "txid", true, "The transaction id for the send. Only 1 transaction is created regardless of the number of addresses."},