If they are still in the probation period and haven't signed an employment contract then I don't believe you are legally bound to pay them any severance but IANAL and the rules vary widely by jurisdiction. Check with an employment lawyer before doing anything. You won't regret spending that money.

I would not recommend trying to buy yourself out (i.e. pay to relieve your guilt). Save the cash for your business and do only what your lawyer says you have to do. There's a chance this employee will see it as a sign of guilt or weakness and try for more. It happened to me.

If this employee does good work but is toxic to the work environment, you might want to suggest a contract arrangement instead of full time employment. You could pay him to take on well defined tasks (offsite) and keep the interpersonal interaction to a minimum. That way you keep the talent but lose the friction.

Add "good communication skills" and "plays well with others" to your job description for the next round and tighten up your employment contract to provide for a probation period where any party can terminate within the first N days. You write up the conditions but get an employment lawyer to help you draft it into something legally enforceable in your jurisdiction.