Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
To work as a collection agent would be halal if the agent was only collecting the principal (the original amount of the loan or whatever was not paid on time).
Answering your question, The Assembly of Muslim Jurists of America, states:
The job of a collection agent is to collect debts and resolve outstanding balances on behalf of a client. The client could be a business, a government agency, or an individual.
In North America, loans and outstanding debts almost always involve interest (Riba).
To work as a collection agent would be halal if the agent was only collecting the principal (the original amount of the loan or whatever was not paid on time), but because collection agents collect interest on late payments as well, this makes this job impermissible.
Read also:
Can I Pay Off My Debt In a Different Currency?
Can We Give Zakah to Prisoners in Debt?
Can I Deduce My Debt from My Zakah?
Is Zakah Due on Properties That Are in Debt?
Is There Zakah on the Debt Someone Owes You?
Is It Permissible to Repay a Debt by Donation of Reward for Fasting?
Almighty Allah knows best.
Source: AMJA Online