Budget reduction column

I have a spreadsheet that I need help thinking through a formula on. Thank you in advance.

I have a single sheet that has 3 different departments all with different budget totals. I like to keep the column blank until an amount is entered, and this formula works perfectly for my needs of keeping a running remaining budget total:

=IF(OR(ISBLANK(F4),ISBLANK(F4)), "", G3-F4)

The issue is that when a different department spends money, then it creates a blank line in the other two departments, so now it returns an error because the line immediately above is now blank. I can not refer to the original budget amount because then it's not a running total of my remaining budget for that department. I need it to refer to the remaining total from the last line that had an expenditure.

I hope this makes sense.

Thanks.

Visual of what I'm wanting to do and how it's not working.

Attachment 838618