At the official start of a matchday (usually Fridays 20:30) your lineup is
lineup is frozen for the evaluation and your account balance is checked. If your account balance is positive, you will receive points for all players you had lined up at that time. An adjustment of the lineup after the official start of the matchday has no effect on the current matchday, but only on the next one.

This also means that you may adjust your squad after the official start of the matchday. The sale and purchase of players will only have an effect on the next matchday. Example: You have Manuel Neuer in your lineup on Friday at 20:30 - Then you can sell him e.g. on Saturday without impact on your points account.