When user makes an in-app purchase, I want their tokens count to decrement by the cost. This will just delete the user's tokens

(This code used to work and broke recently)

    var senderRef = db.ref(`users/${sender.id}/tokens`)
    var amount = 1000
    var tokensCount =  await (await senderRef.once('value')).val()
    console.log("SENDER TOKENS: ", tokensCount, sender.id)
    await senderRef.transaction(async function(currentTokens) {
        var newAmount = currentTokens - amount
        if (newAmount >= 0) {
            return newAmount
        } else {
            return 
        }
    }).then(async (result) => {
        var tokensCount = await (await senderRef.once('value')).val()
        console.log("SENDER TOKENS AFTER TRANSACTION: ", tokensCount, sender.id)
    })

I have data structured as: