(b) An assignee who becomes a limited partner is liable for the obligations to make contributions and return distributions as provided for in this article, provided, however, that the assignee is not obligated for liabilities unknown to the assignee at the time he became a limited partner and which could not be ascertained from the partnership agreement and provided, further, that the assignee is not obligated for any accrued liabilities of the assignor at the time of assignment unless the assignee specifically assumes such liabilities.