Money back guarantee is a promise to refund if a customer is not satisfied with the product or service bought. The way I can think of this moment to express it in Esperanto is "garantio de mono redonado/repagado". Is there a better way to say this?
The phrase you give is not grammatical because of the two consecutive nouns in the same noun phrase thing. You'd need something like “garantio de repago de mono”.
The Dutch ‘equivalent’ (it kind of sounds less formal) Niet goed, geld terug (literally: “Not good, money back”) becomes, according to my Dutch-Esperanto dictionary,
Repago garantiita en kazo de malkontento.
Though I just thought of “Ne bonas, ni monon redonas!” if you want to go for a bit less formal, more advertisementy phrase. (Though the tenses aren't completely right, since it should be redonos, but then it doesn't rhyme.)