Hi,
There is no option in standard to restrict the PO quantity change after posting 541. In fact, there need not be a reference between PO and the 541 movement.
You may check the below options:
1. Create release strategy for subcontracting PO and make the changeability indicator as 1 (Not changeable). SO, once the PO is released, no changes can be done. - But the user can not change the PO after PO release, even before 541.
2. Go for development using BAdI: ME_PROCESS_PO_CUST to validate the vendor stock or 541 movement document and issue message accordingly while changing the PO quantity. (Please note that the 541 movement can be created even without a subcontracting PO reference. You can train the users to make the goods issue against the subcontract PO and maintain the PO number as reference field while transfer posting with 541.)
Regards,
AKPT