Service based quantity contract in sap

Hello, In system, we privation to marque quatity based work contract. Based connected interaction we privation to allocate work quanitity PO to vendor connected milestone basis. Reqeust you to suggest however wayforward.

Example

If we make interaction for 2 services.

Service code  Qty

S1                  10

S2                  10

Now, based connected interaction we are creating PO but strategy is not validating work qunaity of contract. System is allowing to make PO much past services qty of contract. Our requirment is however to restric it.

Thanks successful advance.

Regards,

Bhaumik