£20+30+30 sales and 30 refund = is £80 of sales, and while I see your point that the £30 refund may cancel out one of the two £30 sales and therefore you regard your total as £50 rather than £80, matching up refunds to a particular sale is problematic. Particularly if you say spent £73.21 one day in Tesco and £53.56 the next but the £73.21 was for 6 items of which you return one of them so say get £11.97 back 2 days later. It would be a miracle for Monzo to link that £11.97 to the £73.21
I do not think you need to link a refund to a specific transaction, but rather a merchant or category, that we the calculation should be correct you you will have £50 as total.
That been said, it will be usefull to be able to link a deposit to an expence and make it possible to track if your employer has reimbursed an expence.