When creating Sales Rebates, within the Document Output field you have the option to create 'Purchase Invoices' rather than 'Sales Credit Memos'. In order to do this, Business Central needs a way of knowing what Vendor to create the Invoice for. Firstly, it can only do this for existing vendors so you need to ensure they are already set up in Business Central. Secondly, you need to link the Customer and Vendor together. 

To do this, search for "Contacts" and click into the Vendor "Contact Card" for Type 'Company'. This will be the one in bold font as all others are Contacts of Type 'Person'. The current Contact Business Relation for this will show as 'Vendor'. 

In the Ribbon of the Contact card, click into Related then Contact, Select Company and then Business Relations. Because BC will only allow 1 type of Contact assignment within the Contacts, we need to remove it from here so we can link it to the Customer card instead. So here will show a Vendor assignment line. Simply delete the line. Doing this will change the Business Relation to 'None'. 

Now, open the "Customer" contact card. Currently this is linked to the Customer contact but now we need to assign it to the vendor. Therefore, in the ribbon, select Actions, Functions, Link with Existing and choose Vendor. 

In Vendor No, select the Vendor from the Dropdown you want to use as your Vendor Buying Group. 

This will then add the Vendor to this Buying Group Customer contact card, changing the Business Relation to 'Multiple'. This confirms you have 2 contacts connected. When you now create a 'Purchase Invoice' from a Sales rebate that is allocated for this Customer, a "Purchase Invoice" will be created for the selected Vendor you linked it with. 

Things to ensure:

1. You have a Buying Group created, where the Source Type is 'Customer' and the Buying Group Customer No is selected.
2. You have the Buying Group assigned to individual customer cards
3. You have Allocation Type of 'Buying Group' on your Sales Rebates.