The performance obligation is the placement
Under IFRS 15, revenue is recognised when the agency satisfies its promise to the client. For a permanent placement, that promise is usually met when the candidate starts work, not when the offer is signed. Candidates in the UAE often pull out between offer and start, sometimes after a counter-offer or a visa delay, so start date is the safer trigger.
Retained searches are different. If the client pays in instalments for a defined search process, part of the fee may be earned as stages are completed. Read the contract and record the trigger it sets.