In many places you have written 'he was head of the ministry for 8 years, and did not do anything to stop corruption if there was any'. If you have so much time writing all this, do some research, he was secretary for about 4 years. For rest of the 4 years he was a joint secretary, and for your information there are more than 6/7 joint secretaries excluding DGs in different departments who are also joint secretaries.

Working in such a huge ministry, where DGs have their own powers and responsibilities (clearly given by law, which neither minister nor secretary can question about). So, even if you know something has gone wrong, you can not do anything unless you have some solid and direct proof. Have you heard something of discretionary powers, and its a truth that officials in developing country have excessive amount of discretionary power, in some cases. So even if a minister or secretary is 100% clean, he wont be able to maintain a 100% corruption free environment. However, he can remain 100% clean and pure staying within the limit of law and rules.

It seems that you are of a perception that, since Mininistry of Finance is a hub or corruption and scandals, none of the secretary who maintained this position can do so without being corrupt. If you have some elders and seniors who were in govt service or currently are, you can ask about Devendra Raj Pandey, Narakanta Adhikari and Bhanu Acharya. they are all former finance secretaries.