Paying Back Friend Scenario

So a friend has given me 0.01789463 ETH as a loan to help me out a few months ago of this year which was $40.00 worth at the time (amount I needed). I want to pay him back, do I owe him $40.00 worth of ETH crypto of today's market. Or do i need to payback the 0.01789463 ETH. Trying to get 3rd party opinion about it to make sure this is done properly. Have gone over the scenario many times in my head and i've created so many "what if" situations for myself. So yea need a unbiased opinion, thanks.