EDIT 2 : negative balances problem solved yesterday by Bitfinex, without any loss thanks to the range market (it would not be the same story today with BCHABC pumping). -20$ in fact but who cares haha. My balance was 880$. I lost some more because of the fees though (buying BSV and ABC on exchange to transfer it to margin balance)...

But I still want to be refund the contracted amount : 3400$, not only 2750. I shorted BCH from 400 to 250$, I took a risk and I made some profits, Bitfinex should not interfere with my profits. In fact, my profits is technically here. It's just that Bitfinex should have send me 3400$. Everything is explained in the thread. Thanks for help.