In the first mission? When do you ask? In multiplayer?

If its multiplayer, its a server mod - and has been posted in the tutorials forum about how to change it.

If its single player, then its in the M01a.ini file (mission 1 basically), and the first entry of that file says:

[Mission
mission_title = 1
mission_offer = 1
reward = 2000
npc_ship_file = missions\m01a\npcships.ini

The reward is how much you get for your services You can increases that to whatever you like Remember to start a new game for it to take effect.

The mission 1 file is located:
Freelancer\DATA\MISSIONS\M01A\m01a.ini

Enjoy

Maybe it´s meant to change the money you have when you land on Manhatten first time (after the destruction of FP7) which is normally 500$.