For cash sales involving Paypal or any other sort of money transfer app you do not use MPTF or SCM key prices, you need to get a key mini from the forums to figure out what keys were selling in the app you did the exchange in. Luckily there was a mini done close enough to the date of your trade in the ref suggestion here:

https://backpack.tf/suggestion/678575955edc0a1820032139

All you need to do is divide the cash you transfer by 1.60ish which is what Paypal key sales seemed to be around that price and you got your actual value in the trade, which should be 397ish keys that you can round to 400 based on our guidelines!