Contract or agreement limited by quantity not time

We want to optimise pricing for some materials by placing a large order for a fixed quantity, which will be pre-produced by the supplier and we then call off against this 'contract' until the volume is depleted, and then move to the next 'contract'. How long this will take depends on usage and cant be precisely predicted.

We have tried using contracts, but can only make these work for a fixed time period, rather than the flexible time but fixed quantity situation we have.

This seems like it should be a common requirement - does anyone have any ideas how to manage this in SAP?

Thanks for any help you can give 😊

Liz