No, s2Member doesn't do that by default.
If you were just changing the amount it'd be easier, you'd add an amount field to the button's form, but based on that he'd be given a different kind of access. I can think of two options: use JavaScript to change other values in the form based on what they donate before submitting it, or submit the donation to a custom script of yours where you work with the values a bit and then send it to PayPal from there.
You could save the donated amount in a custom profile field for the user and then with conditionals check that amount and restrict access to content based on it.
If you don't know how to achieve that, you can post a job to hire a freelance developer.
viewforum.php?f=37Still, if it's a subscription and they then modify the amount in their PayPal account, I believe you'd have to adjust the donated amount manually in their profile. Or maybe it can be automated via PayPal's IPN, I'm not sure.
I hope that helps.