In the paypal claim dispute it gives me a option to tick a box with partial payment of $100 along with other options eg. pay full refund and dispute his claim.

That option is there only for you to choose = you can do partial refund; once the case is escalated to claim PP will either decide that the buyer has no claim or tell them to send it back.  That is what I meant when I said PP does not do partial refunds.  🙂