My kids play Roblox and get often get gift cards from friends for birthday presents etc. I've always hated that a gift card for $20 can't be redeemed for the equivalent Robux. Instead you get options to purchase Robux and the most you can redeem is about ~$16, so then you have left over cash and have to get another gift card. Dark pattern.
There's an option buried somewhere in there to convert left-over real dollars into robux. They don't make it too prominent else no-one would be sitting there looking at the expensive robux options that offer (mostly, but NOT always!) a better exchange rate.
Scams and grifts from top to bottom.
visit this page and click the bright blue, prominent button labeled "Convert to Robux":
https://www.roblox.com/my/account#!/billing
you will receive the exact amount you would get from the equivalent conversion rate of the closest "package"