Buy a Roblox Gift Card and give the code to the person
Use the gift card code on the Roblox website to add credit
Buy Robux with the credit after redeeming the gift card
Create a private game pass or shirt in your Roblox group or game
Set the item price to the amount of Robux you want to transfer
Have the other person buy the item
Wait for the Robux to clear before using them
Use Roblox Premium payouts if you are in the same group and eligible
Trade limited items only if both accounts have Roblox Premium and trading is enabled