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Now that we know "C1" is a value and not a reference, we can solve that piece of the problem. You originally said the values run in sequence: C1, C2, etc. The Excel function CONCAT can build such a sequence in successive rows. You tell the function what character(s) form the prefix (for example C) and the number of the row above the first occurrence. For example, if your formula starts in row 12, your would call CONCAT("C",ROW()-11). In row 12, this function would evaluate to "C1". As you copy it down, it would evaluate to "C2", "C3", ... in successive rows. Your formula in row 12 would be
=SUMIF('Invoices 26-27'!$F$48:$F$75,CONCAT("C",ROW()-11),'Invoices 26-27'!$J$48:$J$75)
As you copy the formula down, the actual text of the formula remains unchanged. But when the formula is evaluated, the second argument which calls CONCAT will evaluate to the successive strings you want. Change the "11" to the row before your first formula and then copy it down as far as you need.
If your values do not run in sequence, there are other approaches that will let you enter the values only once and select the one you want to use in each of a sequence of formulas.
You also mentioned needing to handle monthly updated. We need more information. Nothing in your previous posts mentioned dates or data segregated by dates. Tell us what data in your workbook identifies what date applies to an entry.
It would be very helpful if you could provide a screen shot with a few rows of data. If the actual data is sensitive, realistic dummy values would work just as well.