If I am understanding what you're saying, the discount will be the discount no matter when the remitter pays - so long as it's within the net 30 period. To keep numbers simple, if an invoice has an amount due of $100 and the net 30 discount is 10% ($10), then the 'real' amount due (during the 30 day period) is $90. total - discount.

So, with the original disclaimer - you can fill in the discounted amount when you initially enter the invoice record. Then you compare pay date versus due date and take which ever amount is correct for that pay date.

So, when you send out the invoice, it can show something like:

     Amount Due:  $100.00
     Amount if paid by (net 30 date):  $90.00

Your invoice table becomes (assuming a 10% discount):

* Invoice Table
CREATE CURSOR invoice(invno int,date d(8),amount n(5),due d(8),discount n(5))
INSERT INTO invoice values(1,CTOD('01/04/2019'),1500,CTOD('01/05/2019'),150)
INSERT INTO invoice values(2,CTOD('15/04/2019'),2500,CTOD('15/05/2019'),250)
INSERT INTO invoice values(3,CTOD('28/04/2019'),1800,CTOD('28/05/2019'),180)

>Dear Experts
>
>I am working on 30 Days Trade Credit and refer to these links
>
>https://blog.apruve.com/the-difference-between-net-30-and-due-in-30-days
>https://crushthecpaexam.com/accounting-glossary/2-10-n-30/
>
>In short:
>
>Net 30 refers to the trade credit offered to a customer for the sale of goods or services.
>Net 30 means that if the amount due is paid within 30 days, the customer will enjoy a 5% discount.
>
>I have this data
>

>* Invoice Table
>CREATE CURSOR invoice(no n(4),date d(8),amount n(5),due d(8),discount n(5))
>INSERT INTO invoice values(1,CTOD('01/04/2019'),1500,CTOD('01/05/2019'),0)
>INSERT INTO invoice values(1,CTOD('15/04/2019'),2500,CTOD('15/05/2019'),0)
>INSERT INTO invoice values(1,CTOD('28/04/2019'),1800,CTOD('28/05/2019'),0)
>
>* recovery Table
>CREATE CURSOR recovery(no n(4),date d(8),amount n(5))
>INSERT INTO recovery VALUES (1,	CTOD('03/04/2019'),	300)
>INSERT INTO recovery VALUES (2,	CTOD('05/04/2019'),	800)
>INSERT INTO recovery VALUES (3,	CTOD('30/04/2019'),	400)
>INSERT INTO recovery VALUES (4,	CTOD('01/05/2019'),	400)
>INSERT INTO recovery VALUES (5,	CTOD('08/05/2019'),	300)
>INSERT INTO recovery VALUES (6,	CTOD('10/05/2019'),	500)
>INSERT INTO recovery VALUES (7,	CTOD('12/05/2019'),	100)
>INSERT INTO recovery VALUES (8,	CTOD('15/05/2019'),	600)
>INSERT INTO recovery VALUES (9,	CTOD('17/05/2019'),	800)
>INSERT INTO recovery VALUES (10,CTOD('18/05/2019'),	100)
>INSERT INTO recovery VALUES (11,CTOD('19/05/2019'),	300)
>INSERT INTO recovery VALUES (12,CTOD('22/05/2019'),	400)
>

>
>
>More Explanation:
>
>https://pasteboard.co/IfGRYud.png
>(Also attached Image)
>
>Conclusion:
>If an Invoice gets recovery within due date then
>I want to fill discount column in invoice table like shown in above image
>
>Please help