When you sum a column how can you set decimal place for the sum value

Check the fields they should be as numbers with 2 decimals. Otherwise use the  format function ... format(number,"0000,00")

format( Number, Format ) converts a Number into a formatted text. As Format a Text is needed:

0 represents a number or 0
# represents a number that is diplayed only, if the number is big enough
. represents the decimal separator (will be formatted according to the country, that means, in the UK and US it is shown as a floating point)
, represents the grouping separator (will be formatted according to the country,that means, in the UK and US it is shown as a comma)

Examples:

format(42.5, "0") results in "42"
format(42.5, "000") results in "042"
format(42.5, "000.00") results in "042.50" format(42.5, "0.00") results in "42.50" format(42.5, "#,##0.00") results in "42.50" format(1042.5, "#,##0.00") results in "1,042.50"

This solution doesn't work for summing a column. If you set the field formula to:

format(Amount, "0.00")

Then the individual items look fine. But it's no longer possible to do a "Sum". Instead, the options are as if it's a text field - e.g. count, first, last, concatenate.

If it is a text field then you need another formula field that will convert it to a number. If you do a Edit Fields and see a T next to the filed then it is a text field. Create a new formula field, call it convert and put something like number('field name'). Now add this new field to your table view and you should be able to sum it.