### 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_
FeeCalculation fee_calc_out;
            CFeeRate fee_rate{GetMinimumFeeRate(*pwallet, coin_control, &fee_calc_out)};
            /
            /
            if (coin_control.m_feerate && fee_rate > *coin_control.m_feerate) {
               throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Fee rate (%s) is lower than the minimum fee rate setting (%s)", coin_control.m_feerate->ToString(FeeEstimateMode::SAT_VB), fee_rate.ToString(FeeEstimateMode::SAT_VB)));
            }
            if (fee_calc_out.reason == FeeReason::FALLBACK && !pwallet->m_allow_fallback_fee) {
                /
                throw JSONRPCError(RPC_WALLET_ERROR, "Fee estimation failed. Fallbackfee is disabled. Wait a few blocks or enable -fallbackfee.");
            }
            CMutableTransaction rawTx{ConstructTransaction(options["inputs"], recipient_key_value_pairs, options["locktime"], rbf)};
            LOCK(pwallet->cs_wallet);
            CAmount total_input_value(0);
            bool send_max{options.exists("send_max") ? options["send_max"].get_bool() : false};
            if (options.exists("inputs") && options.exists("send_max")) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot combine send_max with specific inputs.");
            } else if (options.exists("inputs") && (options.exists("minconf") || options.exists("maxconf"))) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot combine minconf or maxconf with specific inputs.");
            } else if (options.exists("inputs")) {
                for (const CTxIn& input : rawTx.vin) {
                    if (pwallet->IsSpent(input.prevout)) {
                        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Input not available. UTXO (%s:%d) was already spent.", input.prevout.hash.ToString(), input.prevout.n));
                    }
                    const CWalletTx* tx{pwallet->GetWalletTx(input.prevout.hash)};
                    if (!tx || input.prevout.n >= tx->tx->vout.size() || !(pwallet->IsMine(tx->tx->vout[input.prevout.n]) & (coin_control.fAllowWatchOnly ? ISMINE_ALL : ISMINE_SPENDABLE))) {
                        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Input not found. UTXO (%s:%d) is not part of wallet.", input.prevout.hash.ToString(), input.prevout.n));
                    }
                    total_input_value += tx->tx->vout[input.prevout.n].nValue;
                }