What is the difference between the phrase "he was a *mudallis*" and "he used to practice *tadlis*"?

The questioner : Regarding the issue of "he used to practice *tadlis*" and the issue of "he was a *mudallis*." Naturally, some scholars say that the statement "he used to practice *tadlis*" is lighter, or rather, it does not, for example, it does not invalidate the chain of the hadith, unlike our saying "he was a *mudallis*." So what is your opinion, our Shaykh?

The Shaykh : I am not aware of this distinction.

The questioner: It is not there.

The Shaykh : And I am unable to distinguish between them, because "he used to practice *tadlis*" is a present-tense verb, meaning it is his nature to do that. So if you wanted, what would I say to you? If you derive a noun of agent from a present-tense verb, it comes out as *mudallis*.

The Shaykh : I do not know, did you read this or do you think this?

The questioner : No, I read this, our Shaykh.
Al-Halabi : Did he tell you, "Do you mean he perhaps practiced *tadlis*"?

The Shaykh : Let us stick to his statement. I am asking him: Did you read the distinction between the one about whom it is said *mudallis* and the one about whom it is said "he used to practice *tadlis*"?

The questioner : Yes, my Shaykh.

The Shaykh : Or, for example, "he practiced *tadlis*"?

The questioner : No, my Shaykh, what comes to my mind is that "he used to practice *tadlis*" is different from "he was a *mudallis*."

The Shaykh : All right, do you remember the source? I do not think you remember.

The questioner : It is that, meaning, since it was written...

The Shaykh : Do not trouble yourself, say "I remember," or say "No."

The questioner : No, my Shaykh, Allah willing, I will recall it and give you the news.

The Shaykh : And you refuse to say "No."

The questioner : No, no.

The Shaykh : I am satisfied with one "No" from you.