The budget you have not spent in pillar 1 (if your employer offers it) and pillar 2, is called the residual amount. This residual amount falls under pillar 3.
The residual amount in pillar 3 can only be paid out in cash once a year, after deducting a special employee contribution of 38.07%, which accrues social rights, such as pension rights, with the exception of the right to annual leave.
So concretely, if you have a leftover mobility budget of €100 on the 31st of December, you will be paid out €61,93 net.
The balance of the mobility budget cannot be carried over to a subsequent year.
After all, the employer is legally obliged to pay him the balance in pillar 3 in cash once a year. And this at the latest together with the salary of the first month of the following year.