Simplest option to create purchase order on request from a different legal entity

Summary:

My client whose business is asset intensive is yet to implement procurement modules. Kindly let me know the simplest approach for below scenario:

Content (please ensure you mask any confidential information):

1. Requesting BU A belongs to legal entity A
2. Procuring BU B belongs to legal entity B
3. Product will be tracked as an asset
4. BU B pays on behalf of BU A
5. Once product received, the ownership of the asset is with BU A

Client is not interested in a full-scale implementation. With bare minimum and critically needed configs, kindly suggest the best option to create PO based on request from a different legal entity.