asked my current salary
That's none of their business and in may cases confidential information. If it's actually confidential (check your current contract) then just say so. If not you can "prefer not to disclose" "typical in academia for post docs of this field" or something like this.
The main reason for this question is to determine if their is a major disconnect between your current pay and their budget. Nobody is actively looking for a pay cut so it's typically a lower bound. If the budget is less, it's a non-starter. That's obviously not the case for you, so that should be ok.
and expected salary via email.
Give them a range that you already have in mind. Again, at the early stage it's mainly used to figure out whether your expectation and the budget are in the same ballpark. If not, everybody is just wasting time going forward.
I am sure the employer may low-ball the salary later on
Why are sure about this ? I have never low-balled a candidate since it's just not good practice. Hiring and on-boarding takes a lot of time and money, why would I run the risk of wasting all that effort over a few bucks a year. What's mostly the determining factor is compensation fairness. If I thought your skill/contribution would fit somewhere between Alice and Bob, that your offer will be somewhere in the range of what Alice and Bob are currently making.