Subtract value from a max sum

[Kaas]

Hi guys
I have an excel problem I can't solve myself.

A customer save up half an hour each week.
A1 (week 1) = 0,5
B1 (week 2) = 0,5
C1 (week 3) = 0,5
Etc.

A customer can max save up 6 hours in total.

Then I have a row with the spendings:
A2 (week 1) = (blank)
B2 (week 2) = (blank)
C2 (week 3) = 1
Etc.

I can't figure out this:
If a customer save up and dont spend any until week 24. I can get sum to show 6 by using the IF formula.
And thats great.
The problem is that when the customer spend 3 hours in week 24. The result should be 3 hours left.
But it still shows 6 (because he saved up 12 hours before he spent any, But he can only save up 6 hours).

Sorry for my bad english.
I hope I made myself clear.

Can Anybody help me?

Cheers,
Martin