Does anyone have any ideas on how to record a Purchase Order where the Vendor is actually paying us to take the Inventory?

Interesting. Maybe create a customer record for the vendor.. And create a customer deposit against it? Not really sure whether it will be financially correct. Because the vendor might be paying you to sell the inventory. In that case you are providing service to him. So you could essentially create a service item and invoice that item against it. Don't really know your use case, just my 2 cents.

Yeah financially it is kinda weird. My original thought was to receive in the inventory at $0.00 and then complete an Inventory Adjustment to remove it and place it back in at the negative rate.

you can create customer or vendor record first then add other relationship from relationship tab depending on the initial entity type. This way the entity will be both customer and vendor. Then use the @raghav’s option. this way all transactions stays under one entity. My 0.9 cents worth 🤔