Is there a way that for a competent and helpful answer a one time donation can be given using paypal?
How can I setup this?
not in SMF
You could request a mod be made here: Mod Requests (https://www.simplemachines.org/community/index.php?board=79.0)
If you want users to be able to donate money to other users, you could perhaps setup a visible custom profile field for paypal.me -links or something https://www.paypal.com/paypalme/ or Buy Me A Coffee https://www.buymeacoffee.com/
Or both.
EDIT:
Something like this, that "Buy me a beer" icon there is actually a link to buymeacoffee.
And it's shown next to every post.
Fine, sounds nice.
How can I add such a link to profiles of users?
You would need to set up a custom profile field, with something like this. :)
Donation to the person answering, or to the forum itself? The latter is much easier.