We would like to customize odoo so that each customer has several pre-paid accounts, which are available in the POS. The till operator would choose which account to debit depending on the item being sold. So for instance each customer might have a 'dinner' account which is used in the canteen and a 'stationary' account which is used somewhere else. They will be able top these up in another system, and we will import these balances regularly.

Ideally the account balance would be shown in the POS so that the till operator can refuse sale if insufficient funds. I've looked into customising the POS module and it seems do-able, it's more the customer accounts part which I'm not sure about.