If I pay overtime later, will I get more pay?

Hi, I have a question. I'm working as a student employee at a company, and I was placed through a temporary employment agency. My contract states that after nine months, equal pay will take effect, meaning I'll earn the same amount as other employees. I'm currently working 22 hours of overtime and wanted to know if I'll be charged the higher hourly rate if I cash out the money later.

Example: Hypothetically, I earn €15 an hour, and after nine months, my wages increase to €18 an hour. If I get paid for the overtime later, do I get the hourly rate multiplied by 18, or do I get the hours multiplied by 15 because my overtime was at the time when I was earning less?