People can request to buy in for an amount within the set up buyin range, but the host of the game have to accept the buyin amount before the player is seated. If you only want to have one specific buyin amount, just let your players know, and if they try to buy in with the wrong amount, you can just refuse their buy in request and they have to try again with the correct buyin amount.
Rebuys /top offs are the same. Players request it and the host has to accept.
There are several ways to run this as a host, either you can tell all your players to send you money before play through PayPal / venmo, and only accept buy in requests from players who have already paid you. Or you can let people buyin and play the night out with some rebuys /whatever and in the end settle everything up. Depends how well you trust your group, but I would guess it's better to always have people send you money first, and then accept buyin requests based on how much people have sent you.