Start the other way round, you are keeping 1k of 100k you borrowed and never contributed to. So at this point you still owe them both 50k, or if you refuse to pay them back they have to potential returns of 49.5k maximum. They will not get all the money back you borrowed from them.
The 1k each is owed to them anyway, so it doesn't make a difference if you give it them back with the rest of as a separate payment, it's not profit.