The problem with starting with the baseline of "the right thing to do is always to disclose the vulnerability to Slack regardless of how little they pay" is that it perpetuates the exploitation of legitimate and important work by skilled workers. The onus should be on Slack to provide fair compensation, not on people doing this important work to "do it out of the good of their hearts".
Slack as a company had a revenue of $401 million last year and the average payout in their bug bounty program is $1376 (https://github.blog/2018-03-14-four-years-of-bug-bounty/). That's just disgusting.