Indexed Tables / Sort

I want to make a programm to generate bills. the structure is:
keyindex: +   *
billnr : longint
date : date
Partnumber: string

The keyindex ist the main Index (unique).
the Billnr is the Nr. of the bill, as this table contains the content of
the bill "Billnr"
the date is the date when the article was bought (I need this, as it is
a bill over several months)
the Partnr is the Nr. of the article that has been bougth.

example:

1,53,10.5.97,AAA
2,53,10.5.97,AAB
3,53,11.5.97,AAV
4,54,21.6.97,AAC

Billnr and date are secundary indexes.

My problem is the following:

When inserting a new Item in this database it is automaticly sorted ,
first BillNr and Date. In one date I can have diferent Articles and I
want to define the order of this Articles by my self and not
automaticly, that means via drag & drop in the table I want to change
this order of this Items in one date. In the abouve example I want to
put AAV bevore AAB.

How do I make this? The database always sort in his own way.

bye

Alfonso