Subtracting minutes from a fixed time in a cell

So, here's the problem.

I'll need to create a spdreasheet that must be able to do the following.
Insert the task that was done, the date, and how much time was spent to do that task.
The customer have 4 hours monthly that we give them, so we will do tasks for the customer based in this 4 hours. We go there, do the task, like 30, 25 minutes, and will insert on the spreadsheet, the spreadsheet must be able to when I insert this 20 minutes withdraw this minutes from the 4 hours. So far looks kinda simple and can be solved, BUT, the problem is that is monthly, so next month he will have 4 hours again, not cumulative. Next moth he/she will have just 4 hours, and we don't bill them for those hours.

Problem 2, in the same spreadsheet we must calculate this, when the 4 hours finish, we will calculate a 15-minute increment. Example:
4 hours have finished. Now we will do a 12 minutes task for the customer, the formula should count this: verify how many minutes and if is it smaller then 15 will consider as 15 and multiples per 0.75
If we do a task for them, and spend 16 minutes, the formula should calculate as 30 minutes and multiple that for the correct price, multiple for 0.75.

We give new customer a plus of 4 hours during the first month, so the first month the customer have 8 hours free.

I have attached the spreadsheet, so you can check what I've done so far.

Any small help will be good.

What is "hours included left" (Col E)?

Okay, here's what I did
In Col D (D2) =IF(ROW()=2,4/24,IF(MONTH(B2)>MONTH(B1),4/24,""))
This will give another 4 if entering a new month.

In Col F(F2) =IF(ISNUMBER(D2),CEILING(D2+E2-C2,15/1440),CEILING(MAX(0,F1+E2-C2),15/1440))
I allow you to enter values into column E at any time (possible credits to customer)
If value falls below 0, it will show 0 here.

In Col G (G2) =IF(ROW()=2,0,IF(F2=0,IF(F1=0, CEILING(C2,15/1440),CEILING(ABS(F1+E2-C2),15/1440)),0))
CEILING rounds up to next multiple of 15 minutes. Does this work for you?

First format column H to Currency, then in H2

=IF(MONTH(B3)>MONTH(B2),G2*24*45,0)

It gives a total for that month at the end of each month. Does that work for you?

Sorry man to bother you again, but after showing to my boss he said to change just one thing.
Hours left should also have 15 minutes increment. I was looking up what you did and trying to develop my own formula, but I failed
Column H should calculate the billing times, doesn't matter if the month has end or not. I just noticed that.

See attached, when turns the month, everything brake.

Do you want column H to calculate a value for each row or just the last row (even if not the end of month).
For example if row 7 is an extra hour and 8 is an extra hour and 9 is an extra hour, do you want $45 in cells H7, H8 and H9 or do you want $135 in H9 with H7 and H8 empty OR do you want $45 in H7, $90 in H8 and $135 in H9?

Okay, I modified the attachment and equations in Post #4 above. Is that what you are looking for? I use CEILING which always rounds up (i.e. 16 minutes goes to 30, not to 15). If you want rounding, replace CEILING with MROUND.