where I loan my friend non interest purchases that eventually I will receive a cash payment that will allow the spreadsheet to show that purchases are now fully paid and any short falls indicated until next cash payment
A very simple option:

Type =SUM(B$3:B3)in C3, press Enter, click in C3, and drag down the little black square (fill handle) to fill down the formula.
Tested with LibreOffice 6.4.7.2 (x86); OS: Windows 6.1.
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