Income tax is paid on the value of the remuneration you give at the moment you give it (when you give something).
For example, you can currently sell 100 certificates to an employee at the current valuation, and agree that these will unconditionally belong to him/her for the next 4 years (vested), with a portion to be repurchased if the employee leaves prematurely (<4 years) (reversed vesting). The employee will need to come up with the money now. If you provide this money as a bonus, you will pay income tax on it. If you provide a loan to your employee, you won't pay income tax on it. However, the loan must be concluded under business terms.
If you agree that the employee can buy a certain number of certificates each year for the next four years, it must be at the then-current valuation each year. Again, the employee will need to come up with the money.