SAP system allows user to issue material to the vender through 541 movement type (transfer posting -MB1B) without refering to any purchasing document.

It's a standard SAP functionality which you can control in two ways:

1.  Field settings : Change fields settings for 541 to include Purchase Order number field as mandatory in the screen. But, this is a weak control as you can enter a Purchase Order and send any material to vendor.

OMBW - double click 541 -> double click Material Management

2.  Change the IMG settings for movement type 541 and remove the transaction code MB1B as allowed transaction code. Use ME2O only.  If you want to issue excess material to vendor, you will have to change the Purchase Order components.

OMJJ - click the Select all button
              Type 541 for the Movement Type
              Click the Back button
              Click Allowed Transactions
              Delete transaction code for MB1B

SAP MM Reference Books:
SAP Material Management Interview Questions, Certification and Configuration Books

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SAP MM Purchasing

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SAP MM (Materials Management) Configuration Hints and Tips

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