If you're responsible for SAM, then you BETTER be accountable for getting that purchase information; whether you get it from purchasing or whether purchasing -somehow- interfaces with your SAM efforts.
In fact, a successfull SAM program should consider the 'purchase data' feed first... THEN the software inventory data.
But do keep in mind whether 'procurement' enters the purchase information or you do, purchase data doe NOT equate to 'entitlement' rights.
It's still up to you ( either with a snazzy SAM tool or not) to ensure that the entitlement 'values' can be applied in context to usage rights ( downgrade, corss-edition, dev usage,etc). What's the difference betwwen purchase data and entitlement rights? Take an old Adobe Illustrator 'upgrade' purchase for example; you can certianly purchase a license for an Illustrator 12 upgrade, but you're not ENTITLED to use it unless there's a previous FULL license purchase ( as ga gross example). That's the stuff you can't demand of your procurement team to understand and manage, but the purcase team needs to understand that purchase data must be available to the SAM team