You cannot directly send Robux to another person
Buy a Roblox Gift Card and give them the code
Buy Robux for them through a group payout if you own a group and they are eligible
Create and sell a game pass, shirt, or other item to them
Use a private donation game or experience where they can purchase your items
Trade limited items if both accounts have trading enabled and the items are tradable
Use Roblox Premium payouts if you are paying creators through a group or experience revenue system