You only get one shot at a partial refund, so next time the process would be to agree through messages first then send the formal offer.
Since you exhausted the 1 offer, your options are limited. Full refund then ask the buyer to pay back the difference through PayPal? Ask the buyer to close out the request then send them the agreed amount through PayPal? Both options require a lot of trust that the other party will follow through.
Maybe someone else will have some better ideas.