### 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_
/
                        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Specified output amount to %s is below dust threshold.", addr));
                    }
                }
            }
            const bool lock_unspents{options.exists("lock_unspents") ? options["lock_unspents"].get_bool() : false};
            if (lock_unspents) {
                for (const CTxIn& txin : rawTx.vin) {
                    pwallet->LockCoin(txin.prevout);
                }
            }
            return FinishTransaction(pwallet, options, rawTx);
        }
    };
}
RPCHelpMan walletprocesspsbt()
{
    return RPCHelpMan{"walletprocesspsbt",
                "\nUpdate a PSBT with input information from our wallet and then sign inputs\n"
                "that we can sign for." +
        HELP_REQUIRING_PASSPHRASE,
                {
                    {"psbt", RPCArg::Type::STR, RPCArg::Optional::NO, "The transaction base64 string"},
                    {"sign", RPCArg::Type::BOOL, RPCArg::Default{true}, "Also sign the transaction when updating (requires wallet to be unlocked)"},
                    {"sighashtype", RPCArg::Type::STR, RPCArg::Default{"DEFAULT for Taproot, ALL otherwise"}, "The signature hash type to sign with if not specified by the PSBT. Must be one of\n"
            "       \"DEFAULT\"\n"
            "       \"ALL\"\n"
            "       \"NONE\"\n"
            "       \"SINGLE\"\n"
            "       \"ALL|ANYONECANPAY\"\n"