### 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_
{
    if (options.exists("feeRate")) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Use fee_rate (" + CURRENCY_ATOM + "/vB) instead of feeRate");
    }
    if (options.exists("changeAddress")) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Use change_address instead of changeAddress");
    }
    if (options.exists("changePosition")) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Use change_position instead of changePosition");
    }
    if (options.exists("includeWatching")) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Use include_watching instead of includeWatching");
    }
    if (options.exists("lockUnspents")) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Use lock_unspents instead of lockUnspents");
    }
    if (options.exists("subtractFeeFromOutputs")) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Use subtract_fee_from_outputs instead of subtractFeeFromOutputs");
    }
}
UniValue SendMoney(CWallet& wallet, const CCoinControl &coin_control, std::vector<CRecipient> &recipients, mapValue_t map_value, bool verbose)
{
    EnsureWalletIsUnlocked(wallet);
    
    
    if (wallet.IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Error: Private keys are disabled for this wallet");
    }
    
    std::shuffle(recipients.begin(), recipients.end(), FastRandomContext());