modeling advice for table which may have 2 father tables I am developing a ERP for a small cooperative with special needs (that'y why they need a non-general solution).

I'd like to model the following:
A "receipt" can either be incoming or outgoing
For that I'd like to have three tables: 
collection | payment | receipt
So a receipt can belong to a Collection or a Payment

I'd like to model the Receipt table
with id, collection_id (fk), payment_id (fk)
so there would be a 0,1:n relation from both father tables to the receipt table.

Does that make sense from a DB development view?
Or is there another recommended way to model multiple inheritance?

The same question applies to a a document (image, PDF, etc.) - any uploadable resource:
A document can either belong to an activity, a receipt, a project, etc.
I would Simply add a receipt_id to the collection and payment table.

This is a better design (objectively normalised) that allows your model to grow should your requirements change

This would solve the problem for your document model also - activity, reciept, project would contain a document_id, and you could expand your model to include any number of tables containing a document_id.

If you need to differentiate between documents stored in the table itself, you could add a document_type to make that distinction.