A lender reviewing the buyer's mortgage may still care about the size of a credit, since a large concession can affect the appraised value the lender relies on. That is a conversation between the buyer and their mortgage broker, but as a seller you should expect the number to stay tied to a real cost, not an arbitrary figure.
Legal fees for a seller typically run $800 to $1,500 and a home inspection runs $400 to $600, so those are the kinds of concrete numbers a concession usually references, rather than a round percentage pulled out of the negotiation.
Joel Dyck ties every concession request back to a documented cost before recommending you agree to it. Get your hand-reviewed valuation from Joel Dyck.