The buyer's lawyer contacts the seller's lawyer with the documented problem and a repair estimate, and the two sides typically agree a holdback amount from the sale proceeds rather than reopening the whole deal, so possession still happens on the date already set in the offer, even when the holdback runs to $800 or more.
A buyer should avoid negotiating directly with the seller or the seller's agent at the walkthrough itself, since anything agreed informally on the spot is much harder to enforce than a holdback amount written down and held by the lawyers with the funds actually behind it.
Joel Dyck tells every buyer exactly who to call first the moment something looks wrong at the walkthrough. Get your hand-reviewed valuation from Joel Dyck.