Multiplayer Missions Payout

Solution 1:

When you do a mission for the first time in GTA Online, you gain double the cash and also possibly XP. Says early in the game that players will earn more according to their level or something though I've found if that's the case it's if they've used the extra team life. Pretty sure the case is first try, double cash & Xp.

Solution 2:

Higher level players earn more cash and XP due to needing more XP to gain levels, couple this with the fact that your friend might not have completed that mission yet which nets him more cash and XP.

You can replay a mission over and over but you won't get the same initial payout to prevent players from running easy missions for excessive payouts.