My first question is: did you sign up with the guy personally or with REMAX? I guess this an important point....

Secondly, unless the contract regarding the deposit has been made using a notary then the deposit is not enforceable and the prospective buyer can sue for its return. In most cases, like this one it would appear, it is just not enforced.

On what basis is the agent asking for CHF 8'000 though? What were the agency conditions? Is it success-based only or are you required to pay some costs?

And one last thing, it would help if you told us which REMAX agency it was. This would at least give others on the forum a heads up to be wary when dealing with them.

One, very last thing, added as an afterhought: If you are members of the Hauseigentümerverband (owners association), they give free legal advice. membership is only about CHF 50. I know you are leaving, but it may be worth signing up just to get the advice.