Create file metadata input

There are several ways to do this, depending on what are your other requirements regarding the invoice and the related documents:

1. Create the Invoice as a separate object type and then add the Proof of Payment as a document where you tag the Invoice in the document metadata (property definition of type "Choose from list 'Invoices'). Now the Proof of Payment is linked to the Invoice object and you can find it from the Invoice object's relationships. When using this approach you can find all the Invoice documents as separate objects for instance in search results and these documents can have their own metadata, workflow, permissions etc. independent of the Invoice object.
2. Alternatively you can attach the Proof of Payment directly to the Invoice object if you allow the Invoice object type to have files (you can enable this in the object type settings: "Objects of this type can have files"). This would work if the Proof of Payment can have the same metadata, workflow, permissions etc. as the Invoice object itself and you don't really need to find the Proof of Payment separately for instance with search.
3. The third option would be to have the Invoice as a document class instead of a separate object type and convert the document to a multi-file document when you want to add the Proof of Payment. Now both the original invoice file and the proof of payment file are found in the same multi-file document and share the same metadata, workflow and permissions.