Fixed-price contracts are based on a fixed scope, which is not compatible with Agile. However, most customers still insist on having fixed-price contracts, and since many suppliers cannot afford to lose that kind of customers, it's common to see Agile projects that cop with fixed-price contracts.
The general approach is that the size of the elements of scope are estimated. When a new feature is requested, the customer will "swap" or "trade" it with one of the existing features with the same size.
While having time and material contracts are preferred in most Agile frameworks, PRINCE2 Agile only considers the fixed price contracts.