(1) A person who is admitted as a partner into an existing firm does not thereby become liable to the creditors of the firm for anything done before he became a partner.
(2) A partner who retires from a firm does not thereby cease to be liable for partnership debts or obligations incurred before his retirement.
(3) A retiring partner may be discharged from any existing liabilities, by an agreement to that effect between himself and the members of the firm as newly constituted and the creditors, and this agreement may be either express or inferred as a fact from the course of dealing between the creditors and the firm as newly constituted.