Question

Is the categorical imperative a proof that an act is immoral, or is it that the very reason that if everyone behaved in a certain way harm would result is what makes the act immoral?
I had an argument about this, because there is a claim regarding the sale permit (heter mekhira) that a person cannot choose to be stringent for himself and not buy, because if everyone refrained from buying the growers would collapse, and that is immoral according to the categorical imperative. But in reality there is no concern that there will be no buyers, so the stringency itself is not immoral; only if everyone behaved that way would it be immoral. Is that included in the categorical imperative?
I’m also not convinced that there is a difference between the possibilities, and I would be glad for the Rabbi’s answer.
Thanks in advance