Hi lb,
Pie charts show values totaling 100%. You start off with 100, or 100% available and 0 or 0% taken away.
Each $10 entry subtracts 10 from the original 100. So after the first entry and subtraction you have 90 available and 10 taken, adding up to 100%.

As the entries continue, the green wedge grows larger. Here seven entries have been subtracted from the available funds. I've shown this image with the chart selected to show where it is getting the two numbers, and the labels ro the legend below the pie.

After the tenth entry you have subtracted all 100 and have 0 available, leaving a 100% green chart.

I used formulas to calculate the amount left in column B, and to keep the cells blank where an amount to be subtracted had not yet been entered in column C.
B3: =IF(C>0,$B$2-SUM($C$2:C3),"")
Fill down to B12.
Formula in B13 and C13 calculate the amounts to be displayed on the chart.
B13: =MAX(B2:B12)-SUM(C2:C12)
C13: =SUM(C2:C12)
To change the chart colours from the default blue and darker green, select the chart, then click on the coloured square above column B to select the 'blue' wedge, then click the Fill Color well in the format bar and choose a red from the palette. If you want a brighter green than the one assigned by Numbers to column C's part of the chart, click th coloured square above column C, and repeat the process.
Regards,
Barry