@lubos has an idea to implement purchase work flow and sales workflow, which will have order balances. I have been pushing for this for a very long time now because in my company and many other companies in Ghana we do progressive invoicing to our customers based on POs. So the balance on POs must always be known. We also Send POs to our suppliers some of who also do progressive invoicing.
Check this thread Managing Sales and Purchases Orders (Advanced Features Request/Suggestion) - #12 by ShaneAU Read to the end
A feature that will enable tracking of the balances on orders (Sales/purchases) is much needed for the management of sales and purchases.
On the RFQ I think the opposite is the sales Quote which already exists.