SAP/MM : contract

Our process has the following steps - The total amount in the order to the vendor is fixed. Indivdual services have rates fixed, but quantities are not fixed ie, the quantity can vary from 0 to any upper number subject to the overall value of the order. I tried the following:
Created a contract (ME31K) with the services and rates entered (quantity kept as 1). Overall value entered as the contract limit. PO created (ME21N) and Contract details and overall limits entered. Service items adopted from the contract and the 'No Limit' box checked for individual services. All service quantities maintained as 1 no, rates copied from contract.
With this my expectation was that the system will allow multuiple service entry sheets for each of the services upto the limit specified by the contract. However the system allowed Service Entry for value exceeding the overall limit of the contract.
What other configuration needs to be done to make sure that beyond the overall value limit, system does not allow service entry.