Can I pay off the full amount of an instalment plan early?

You always have the right to pay off parts of or the entire debt before the agreed due date (settle the debt). If you choose to settle, you should then pay interest and other costs for the credit for the time up to the early repayment but not for the time thereafter.

The lender may not demand any compensation for you paying early. However, if you have agreed on a fixed interest rate, the lender has the right to demand compensation for lost interest income, so-called interest differential compensation.