Hello Weiching!
It's true that we don't have the Amount (LCY) field available in the Sales/Purchase documents (headers) to be selected in the On Condition.
Maybe if you operate with different Currencies and you want to control different Amounts for the different Currencies for Approval, you could add 1 workflow per Currency in your side.
The trick would be to add the Amount filtering as usual, but filtering also by "Currency Code" as well, so you specify the right limited Amount per Currency to be controlled by your Approver(s).
For example for "Currency Code" = USD and Amount > 100 dolars

Then you can create another one and filter by "Currency Code" = EUR and Amount > 85 €

Only the Workflow where all the On Conditions are met will be executed, so you will run just 1 at a time, the relevant to the document you send for approval, depending on the Currency as well.
I hope that helps :-)