Is it permissible, Islamically, to file a lawsuit against a sales representative who failed to collect the financial outstanding debts from his clients after his resignation, knowing that a portion of his salaries is withheld pending the resolution of this issue?

The contract between you and the sales representative is an agency contract for a fee. An agent is trustworthy and is only liable in cases of transgression or negligence. He is obligated to collect the prices of the goods.

If the representative neglects to collect the price, he is liable for the goods.

If he is unable to collect the price without negligence, you cannot hold him liable. You are permitted to take from his money that you possess an amount equivalent to your right if you are unable to recover it from him.