swapTokensForExactETH, execution reverted: null

Question

I m trying to estimate gas for a swapTokensForExactETH transaction with pancakeswap contract (similar to uniswap one)

When i do, i get the following error: Error: Returned error: execution reverted null

After some research it appears i shouldn't give the value field when building the transaction with this contract method. But when i don't provide this field, i get the following error: Error: Returned error: execution reverted: PancakeRouter: EXCESSIVE_INPUT_AMOUNT, same goes if i put 0 in value

For reference:

• Using Binance Smart Chain
• 0.00024 BNB = 1 Token.
• I have 10 tokens in my wallet + 1 BNB.
• I have approved the token on Pancakeswap
• I did manage to use swapExactETHForTokens successfully already, but not swapTokensForExactETH yet

// For your information:
// `CNF.wallet` = My wallet address
// `config.token.contract`= `0x00e1656e45f18ec6747f5a8496fd39b50b38396d` (BCOIN)
// `config.crypto.bnb.contract` = `0xbb4cdb9cbd36b01bd1cbaebf2de08d9173bc095c`
// `config.crypto.pancake.router.contract` = `0x10ED43C718714eb63d5aA57B78B54704E256024E` 



var amountToGet = 0.00024; //exact amount of BNB that i want (equal to 1 token)
var amountOutMax = 5; //max amount of tokens i m willing to use to get above BNB amount

var data = contract.methods.swapTokensForExactETH(
    web3_bsc.utils.toHex(amountToGet), 
    web3_bsc.utils.toHex(amountOutMax),
    [config.token.contract, config.crypto.bnb.contract],
    CNF.wallet,
    web3_bsc.utils.toHex(Math.round(Date.now() / 1000) + 60 * 20), //deadline
);

var count = await web3_bsc.eth.getTransactionCount(CNF.wallet);
var rawTransaction = {
    "from": CNF.wallet,
    "gasLimit": web3_bsc.utils.toHex(CNF.gasmax),
    "to": config.crypto.pancake.router.contract,
    // "value":web3_bsc.utils.toHex(amountToGet),
    // "value": 0,
    "data": data.encodeABI(),
    "nonce": web3_bsc.utils.toHex(count)
};


web3_bsc.eth.estimateGas(rawTransaction, (error, gasEstimate) => {

   if (!error && gasEstimate) {
   
        //continue...
   }
   else {
   
        //error
   }
   
});

Answer 1

You forgot to convert the amounts from ether to wei.

https://web3js.readthedocs.io/en/v1.2.11/web3-utils.html#towei

Amounts sent in ethereum contracts are uint256, you must convert your floating point ether value to wei unit, which is done by multiplying by the number of decimal points a token has.

WBNB and your token are using 18 decimals, so call

web3_bsc.utils.toHex(web3_bsc.utils.toWei(amountToGet)),
web3_bsc.utils.toHex(web3_bsc.utils.toWei(amountOutMax)),

Your amountInMax is "5", this means 0.00000000000000005 BCOIN, hence EXCESSIVE_INPUT_AMOUNT is thrown.

---

swapTokensForExactETH swaps an unknown amount of Token into exact ETH.

You shouldn't specify value since you're receiving ETH (BNB in this case) and value sends ETH along to the contract.

According to docs -

function swapTokensForExactETH(uint amountOut, uint amountInMax, address[] calldata path, address to, uint deadline)
  external
  returns (uint[] memory amounts);

• amountOut is the amount of ETH to receive.
• amountInMax is the maximum amount of input tokens that can be required before the transaction reverts.

The EXCESSIVE_INPUT_AMOUNT error means that to match your requested amountOut (exact ETH), you must pay too much of Token, more than allowed in amountInMax. This means you're not calculating enough slippage in amountInMax to sustain normal price fluctuations.

If you want this to work in real time, you should use UniswapSDK to calculate inputAmount using the Trade object, then add 0.1% for slippage and use that for maxInputAmount.

Make sure you have enough of Token in your wallet to cover this.