Probably this is what you need. In the Textbox.lostfocus add:

IF this.Value>1000
	this.InputMask="99,999"
else
	this.InputMask="9999"
endif
this.refresh

>Hi
>
>I have a text box with inputmask='99,999' I want the comma only if the amount is > 1000
>
>thanx