Several things that you might want to explain further:

1. How does asking for OP [and his client's] telegram details fit into this investigation of the source of fund?

- Information from the messenger may be indirect evidence of honest receipt of funds and disclose some details of the source of funds.

2. Why do OP has to be [initially] the one asking his client to provide details and how is it not counterproductive toward an investigation of the legality of fund like what I pointed out here?

- In this case, the OP is not the sender of the funds for the order. According to our rules, the sender of funds must provide information about the source of funds.

3. How is what he provided from his side is not enough? He proves that he worked for it, got them as a payment. Unless proven otherwise, it's all legal from his side in a sense that he's not involved or have any idea in the said criminal activity.

- Since the OP is not the sender of the funds, he has nothing to do with the funds sent to us. If the user had received funds for any service, we would have requested details of the transaction.

4. How do you think OP will know that the fund is related to criminal activity instead of being a pure innocent here? Wouldn't considering him ineligible to prove the source of fund as he's not the sender rather contradict the assumption that he's being involved in the activity [which become the reason why you hold his fund, because he knows it's tainted, otherwise, he's clueless, innocent, and the fund should be released to him]?

- The availability of complete information about their own counterparties, as well as checking funds for a possible connection with a crime, is a direct responsibility of the services engaged in the exchange of funds. In this case, the OP explicitly said about providing money exchange services on an ongoing basis.
As we have already said, in this case, the OP has nothing to do with the funds sent to our addresses from a legal point of view. When accepting criminal funds to their own addresses, there are other legal consequences for users.

5. How much from that fund is marked as tainted? All of it?

- Yes, all the funds sent are related to criminal activity. We cannot disclose the details of the incident publicly, as the disclosure of such data may adversely affect the investigation