changing selection screen values

In the selection screen, there are 2 fields, 1 is vendor ID and 1 is vendor name:

Parameters: s_venID like lfa1-lifnr,

s_venNm like lfa1-name1.

The basic requirement is that when user enters the vendor ID, the vendor name could be retrieved and displayed on the screen.

So I wrote in the At selection-screen event:

At selection-screen.

select single name1 into s_venNm from LFA1 where lifnr = s_vendID.

It works fine whenever I change the vendor ID on the screen.

The further requirement is that user could also change the vendor name on the screen.

When I change the vendor name and press enter, the name would load from the vendor ID again.

How could I avoid this?

Thanks!