[Bug] No proper check in buildTX

[0xNegnin]

xchainjs-bitcoin/utils buildTX needs to check that the amount sent + fee cannot exceed the users balance, if it does then the amount sent must be equal to the users balance - fee, it should not throw an error but instead determine the new implicated amount to send (as it is maximum in this context).

[mikewyszinski]

@0xNegnin sorry about the long delay, looking at this now...

under the covers, we are using https://github.com/bitcoinjs/coinselect/blob/master/accumulative.js to select the inputs based on the desired outputs in the tx. if not it will error out.

I think sending the entire balance in this case is not a good idea as that's not what the caller requested.

Could a workaround be the following?

1. make your call to transfer()
2. if it throws Error('Insufficient Balance for transaction')
3. you catch, check balances and possibly resubmit if it makes sense?

[github-actions[bot]]

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