BUX guarantees that you cannot lose more money than the amount that you invest. We have taken two precautionary measures for this:
1) We will close your position the moment you have lost 100% of your invested amount. This is also referred to as a "guaranteed stop-loss".
2) We are also "reserving" a certain amount of cash in your account when you open a trade (see: What is the Ring Fenced amount?). This amount is reserved to cover possible closing Fees and/or the Financing Fee of your open trade and protects your account balance. And when you close all of your positions, this amount will be free for withdrawal again.
All in all, these measures make sure that you won't have to cover unexpected losses and that your account will never drop into the negative.
Let us know if you have any questions at BUX Support.