Why does system show the message ‘Net amount does not match paid amount’ in direct payment?

Direct Payments is a payment entry indicating that payment has been made for a Purchase receipt or any other payments. Basically, Direct Payment lets you pay your bills or tuition, donate to your favorite charity, make a Purchase, or send money to a friend or family member electronically from your checking or savings account.
Users can select the payment type cash /bank, sundry type as others, sundry code, payment mode, Purchase order number(optional).
cheque/dd number and date are mandatory if payment mode is cheque/dd.
In details, Account details should be mentioned with the amount. Once amount is entered net amount is header is populated.
Users can finish the payment by clicking ‘paid’ from the more option of the record in feed view. Once paid then in the record paid amount gets populated. On confirming, If the amount paid is not equal to net amount then system will show the error message ‘Amount incorrect Net amount does not match paid amount’.