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The held funds are unavailable until PayPal releases it so if you issued a refund, it will come from your PayPal balance or your bank account if your PayPal balance is barren. Then after 21 days the held funds will be released to you.
Once a refund is issued, I don't believe the buyer can file a dispute for the held funds because they can't double dip.
I sometimes wonder if one asked the buyer to open a dispute, would PayPal use the held funds to refund the buyer with or it is still unavailable and they still take the funds from your PayPal balance/bank account. One would read different things happening in situations like these and there's no help documentation explaining these situations.