When adding an item to a shared tab or splitting a bill, the user making the request should have the options (perhaps an on/off toggle) that allows the recipient to choose the amount they contribute.
Use case example: 6 people go out to dinner, to save the card machine song and dance, one person pays and splits the bill using Monzo. If they’re the type of crowd that don’t want to split the bill evenly and just want to pay for what they ordered (no judgment!), right now the original person who paid would have to add in all of the amounts for all 6 people. With this feature, they would be able to choose to split the bill and toggle on ‘recipient chooses amount’. When a recipient receives the request, they accept and choose the amount they want to contribute.
I could also be useful if, when splitting the bill and adding a receipt, others with whom the bill is split see the receipt (currently they do not).