Yes. You may pay ANY amount.
When you click to pay, you may edit amount.
If you want to pay less than the full amount of the milestone, the freelancer will be asked to confirm.
I'm sorry but this information doesn't seem to apply to my case. I don't have a button "Review & Pay". I have a "Pay" button and after I click it I cannot edit the sum to release. I can only click on "Pay".
Once you're in your Contract room, please select the ''Milestones & Payments'' tab, then click on the ''Review & Pay'' button. There, under the Payment section, click on the ''Approve & Pay'' button which will open the pop-up window for you where you can use the pencil icon to edit the amount you'd like to release to freelancer.
I hope this helped.