This page will explain the system's behavior regarding the usage of contracts in Purchase Orders (PO) with limit items.
Besides the explanation about the system's behavior, a note will be mentioned for better understanding of the reason of this behavior.
The use of contracts in limit Purchase Orders (PO) is not supported in the case of extended classic scenario. This behavior is same from earlier release.
The reason for that is the creation of follow on document. For example: in case a goods receipt/invoice is posted, then the call off (update of contract release value from PO value) from those follow on documents is difficult because goods receipt/invoice reads only the backend PO and not directly the leading PO in SRM. This is a technical limitation which is not yet supported.
Check note 1358679 for more information.
SAP Note: 1358679 - Contract Limit fields enabled in extended classic scenario