The seller of an account is charging me more than the listed price after I sent them the money

I paid 70 dollars in steam gift cards as per the seller's request for the 70 dollars account. He then demanded another 30 dollars if I want the account. I refused because I was not made aware of this before I already paid. He then offered a lower price of 20 dollars more to which I and he agreed to be the last payment before I receive the account. After I paid he did not respond back. I have evidence of our chat and of my purchase.