What about adding secondary, user-modifiable data-points to a transaction (similar to being able to add attachments to it) without affecting its main important data but having an effect on targets?
I know this would complicate things, but could be a solution to this problem.
The amount field on all records stays always the same, non-editable, having no effect on balance. A data-point “Percentage responsibility” (or whatever you want to call it) could be added where the user can manually (maybe with the help of a slider) choose a value between 1 and 100, but default value is always 100. This value would then be multiplied by the amount and contribute this figure towards the targets.
Bill splitting and monzo.me could be integrated to these editable fields.
Example: I settle the £200 dinner bill but I am one of four, I would want to manually go to this detail transaction editing screen and slide that value to 25 (if there is four of us), then targets will only go up by £50 but for every other basic account data (transaction feed in app, statements…), the transaction remains a £200 restaurant payment.