You cannot use formulae in the cells to do what you'd like as you will
create a circular reference. You already know this, of course.

You could put your daily sales figures into 31 columns in another sheet
and refer to them from your MTD cells, if you have the columns after C
populated with other information. (I'm assuming 31 sales days. If your
sales are on a Monday-through-Friday basis, you won't need as many
columns.) This has the advantage of enabling you to correct mistakes;
if you put in sales of, say, 6378 instead of 3678 for a particular day
you can see that later on and enter the correct value. Doing it the
way you describe means that the MTD figures cannot be easily
reconciled, and you have to trust yourself to always enter the correct
figures. I don't know about you, but I wouldn't ever trust myself to
do that. We're only human.

To get the behavior you describe would take some VBA code that
determines when a number has been entered in a daily sales cell, adds
that figure to the MTD figure, then deletes the daily figure. With all
due respect, it's far easier and much more transparent to do all this
calculating right on the spreadsheet. And, as I stated above, far
easier to correct errors in data entry.