TypeError: Cannot create property ’negative‘ on string ‚1234‘

The reason is that you either passed an argument of wrong type and not BN or a bracket mistake

This is correct:

const ethStateBeforeInvesting = new web3.utils.BN(await web3.eth.getBalance(investor))
const working = ethStateBeforeInvesting.sub(ether("1").sub(new web3.utils.BN("1000")))