It's not imposing, it's being assertive. Girls like guys that are more assertive and go after things. It provides more certaintity and communicates to girls that he knows what he wants. Girls don't like flakey, uncertain, wishy washy behaviour from guys even if that is how they are too.
Girls are going to respond more favorably to someone asking: "Wanna do something this Thursday?" vs. "Hey, get back to me whenever you are free".
In this case, he's just merely asking a question. It's not like he said "I want to see you next Saturday then." That to me is imposing.
Just calibrate yourself so that you are assertive but not a push-over as you don't wanna freak out or scare the girl in anyway.
If she thought next weekend would be fine, she would've said so on her own. Saying "what about next weekend?" comes across to me as socially incompetent, needy, or both, pressing her for availability out of either not understanding or not caring about the social cue at play here.
And why would you be in a rush to see her and try to book her up far in advance if she isn't in the same rush to see you? On top of that, if you actually thought she herself wanted to see you, there would be no need to try to press her to do so as she would hit you up later herself. So the fact that you do it just adds to the cringe factor of social incompetence and/or neediness, you don't trust that she wants to see you and you basically want to obligate or guilt trip her into booking a time in advance.
That's how I imagine it, whatever works for you.