A contract based on outcomes or throughput needs collaboration between the customer and the supplier.
In simple terms an agile contract should create the right behaviours; moving from a solution-based view to a view based around throughput or outcomes is more likely to produce those beneficial behaviours.
Structuring a contract to align with the agile way of working is advantageous when using PRINCE2 Agile because the customer and supplier should be constantly responding to change in order to deliver the highest value possible in the defined timescale. This therefore needs a contract that supports the objective of delivering value and allowing for change.
Ultimately, the contract should be seen as a safety net and not a weapon.