Is the money I obtained by mistake from my partner, which he was using to negotiate his exit from the company, permissible for me or impermissible, and must I return it?

The company is a non-binding contract. Whoever among the partners wishes to dissolve the partnership may do so, and he shall bear the losses incurred in it according to his share, just as he would receive a portion of the profits according to his share as well. The demands made by the partner in return for his withdrawal are void. He must bear his share of the loss like the other partners, in addition to any loss caused by his transgression or negligence. If he refuses, it is permissible to take what is due from the available funds.