Before handover the sequence of payment and possession must be settled.
Without a clear link between handover and final payment you as seller give up possession before the money is secured, or you as buyer pay without receiving the apartment. Set the handover date in the contract and tie it to the final payment or to the fulfilment of the escrow conditions. Our checklist on handover and vacating offers guidance.
Only with a settled sequence can the handover be carried out without payment risk.