I found the answer :-)
Problem with the other 2 solutiuons were (sorry guys a bit vague in my
question) that if the amount was 1234.56, it should show as [1234.56] in
stead of [ 1234.56].

The answer is :
="[" & TEXT(M3*1.1,"___0.00") & "]"

"Johan" wrote:

I have a the following value in a cell: [ 123.45]
I need to add 10% to the value and preserve the formatting.
I've split the value(123.45) into another cell(M3)
Thus, the formula would be : ="[" & TEXT(M3*1.1,"0.00") & "]" which results
in [135.80].
The answer should have looked like [ 135.80]
What should the format_text value have been?