Buy a Roblox Gift Card and give the code to the person
Buy Robux for their account if you have access to it and they allow it
Create a group and use group payouts to send Robux to members
Make a clothing item or game pass and have the person buy it
Trade limited items with another user if you both have Roblox Premium
Use the Roblox marketplace to transfer value through item sales, if allowed by Roblox rules