Markus Maybe a simpeler solution/implementation of the feature:
My 2 cents:
Allow the creation of a proxy subaccount (that can be linked to cards like now already and dualpin etc) choose a max amount or select other account and calculate the total allowed amount/balance on the proxy subaccount. Preventing to 'overspend'. Just an account viewpoint instead of a card one.
Allow you to assign the transaction to the correct on proxy subaccount later. (Let's say within a max time window otherwise a default subaccount is targeted)
Issue (think this exists with curve as wel)
One drawback/challenge might be how the iban on the other party of the transaction is handled, because the proxy iban vs target account iban. Especially when use case 2 is in play or other proof of payments.
Use-cases:
no 'mistakes' on the accounts purpose (clothes on groceries example)
Seperate business travel expenses and personal ones without mistakes so that the 'ask-employer-to-reimburse' account/transaction overview can be shared/targeted without private transactions. (Also more generic, when the targeted account is a connected one shared with you.