Yes, after some emails back and forth it was today that I heard they agree to my offered solution: they pay me the full 399.65mbtc outstanding balance, but in increments of 2000 euro per month.
What do you mean they pay you 2000 Euro per month?
The instalment they pay you varies on the current BTC price?
So if BTC is skyrocketing, you will lose value?
I mean, if that was your suggestion, it's alright. Maybe you planned to sell your BTC anyway, so it wouldn't make much of a difference.
Anyways, congrats!
I am happy to say that Buff afterall are not thieves, they just (...)
Wrong. They are thieves who got caught and then being rueful

They always had a 3000 euro equivalent monthly limit, now I suggested they can make it 2000 if they want.
It does not mean I will lose value. My BTC balance remains the same. Just every month they calculate at the moment of payout what 2000 euro is worth in bitcoins and pay me that.
I will still receive the full 399.65mbtc no matter what happens. If BTC skyrockets it will just take longer for me to receive that full amount, and if Bitcoin price falls deep it will take a shorter time for me to receive the full amount.
And no, you're wrong about the sell-assumtion. I'm not planning to sell any of my BTC in the coming years at all. And exactly this solution suits that need. I keep my balance in BTC and receive a small part of it each month. If BTC price rises I still take full profit of it just as much as I would had i just kep the BTC in my own wallet all this time.