Payments on account and interest calculation

Hello Experts,

I am in one electricity project.

I came across a new requirement.

The process is as below

1) Suppose consumer electricity bill for month march/16 is $ 294

2) He pays amount $300 in banks on 20th march/16

3) Now we upload this payment from external payment gateway on 21th march/16

4) System clear the bill amount 294 and remaining $ 6 goes on payment on account in FPL9

5) Now the next bill is generated for the next month on 20th April is $100

6) First $6 will be adjusted into bill and the remaining amount $94 will be posted as debit in to consumer account.

Q) I want to calculate the interest on payment on account $6 from date 21st march to 20th April, but system is not calculation the interest because clearing date is 20th April

Kindly suggest.

Regards

Amit