As title says, customers should input the amount they are going to pay with when choosing "Cash payment". This should be a numeric field and the amount should be equal or above the total bill.
I read a similar request that got denied and the "solution" was to "wait for the client to finish the process and start the WhatsApp chat so that then the business can ask the customer how much money they are going to pay with". Of course this is a "solution" but absolutely not the most efficient one. If we are already using TakeApp it's because we want to minimize the interaction with the customer and automate our processes. Such a simple feature should be added within TakeApp.