In an adviser engagement, a retainer may be a payment for ongoing availability or work, or an amount paid in advance towards future charges. The word is used in different ways, so the agreement needs to explain the arrangement.
Check what the payment covers, when it is due, whether it is credited against another fee and what happens to any unused amount when the engagement ends.
Read the retainer with the rest of the charging terms. A credit towards a success fee is different from a separate charge, and the treatment may depend on whether a transaction completes.
Keep the written explanation with the agreed scope and approval process for additional work.