'link cells by subtraction operation

I am looking to connect the buttons to the cells in i3 and cell l3. So that whenever the number in cell i3 is added or subtracted to the number in l3 is always subtracted from starting at 500 dollars. So if 40 is in cell i3 cell l3 should read for 460. If 60 is in cell i3 then cell l3 should read 440. I have added my code below. Its subtracting to much.

Sub Lower()
Range("i3").Value = Range("i3").Value - 1
Range("l3").Value = Range("l3").Value - Range("i3").Value
End Sub


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