There’s 2 scenarios that would work
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Joe set up the pot so therefore he is the admin. All users have put in equal shares. Joe clicks to collect the funds. Each user is sent a push notification to ask if they’d like to let Joe withdraw the money, all users click yes, the money goes into joes account and Monzo avoid responsibility as each user concerned and knew the risk.
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Joe, jack and Gill have a shared pot, joe and jack put 100 in each but Gill only puts 50 and refuses to pay the rest. Joe and Jack are unhappy with this to they want to withdraw. Each user has a cancel pot button they can click at any time to terminate the pot, the total funds each user put in is returned to their respective accounts. Everyone got their money back, win win.
An extension to scenario 1 is once the purchase has been made, all users can click to attach a transaction from the transaction list to the withdrawal. Each user can preview it and if they’re happy click confirm.