unless someone has a better idea

So you would

var total = getTotal()
val list = bankStatement.transactions.map {
    val currentTotal = total
    val totalBefore = currentTotal - it.amount
    total = totalBefore
    Something.fromThing(it, currentTotal, totalBefore)
}

that should work

@menegatti suggested something better:

some.things.map { 
    Something.fromOtherThing(it, currentTotal, currentTotal - it.amount).also {
            currentTotal -= it.amount
    }
}

yeah

tbh, not sure it is better… I get a bit cautious around

also

because it might hinder readability

My version was very ugly imho. I like yours a lot more