CPA Ties to PO Automatically

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When creating a PO and tying a master contract to it, the CPA automatically ties to the PO--which is what we want. But the CPA automatically ties to POs even if the master contract IS NOT associated to the PO. There are times the purchase is not on the contract, therefore we do not want the CPA automatically tying each time. The only time we want a CPA associated with a PO is when there is a Master Contract tied to the PO. Is there a way to force this? Or a way to stop CPAs from automatically tying to