So I looked back at where I got this from - an episode of Cool People Who Did Cool Stuff about Diogenes - and it seems this is more of an interpretation of Diogenes' teachings by Margaret (the host) rather than something he said directly. Here's the relevant portion:

Speaker 2 (27:57): Because he also wrote books, so he clearly owned more than and just these things. You know, at various points, maybe he's like borrowing a pen to write a book. I don't know whatever, And basically he's like, hey, sell me to that guy, and people are like, oh, it's because he believes so much in this philosophy. I think Diogenes is being canny and knew it would go better with that guy, probably because he knows he's educated, like

(28:20): he himself is educated, and he's like, oh, I can get myself into like a teaching job instead of a fucking mining job, you know, right. He works for Zeniades for a few years, running the man's household and tutoring his kids, who apparently all loved him, and then he was freed. Letting people buy their own freedom was a

(28:41): way to recoup capital costs. You steal a few years of someone's life and then you let them pay you back. Your upfront cost. And also if you promise the slave you're going to free them, they're much nicer to you.

Speaker 1 (28:53): Yeah, and you are able to extract their labor.

Speaker 2 (28:55): Yeah, totally. And so it's technically hypocritical of him to want to become free again and to like buy himself this freedom. And I do not blame him for this hypocrisy at all. This is a completely natural thing. He taught that if you desire to better your station in society, like a slave who wants to be free, as the example uses, it'll never be enough. You'll get free, and

(29:15): then you'll want to be a slave owner, obviously, naturally, that just happens to everyone, and then you'd want to be a landowner, and then a citizen, and then an officer, and then a king and then godhood. Where does it stop?

Speaker 1 (29:28): I know, it's just too much slop, very slope.

Speaker 2 (29:31): But like whatever is a philosopher, he's going to make random as philosophical points, and he sharees oak the freedom that was offered him, and he didn't go and buy someone, so good on him.

The transcript is kinda terrible so maybe just skip to this part and listen, but that's where I got it from. There are sources linked in the first episode description but I couldn't find the specific quote about not improving one's station in life after a quick search, so ¯\(ツ)/¯.

I'll see if I can find a source for that and get back to you. I could be misremembering.