Agree with Brit, the laws need to be properly drafted to cover those concerns.

Pilgram, with regards to having a formal agreement, if the law of the land does not support it, the agreement (written or verbal) will be void or voidable and thus, not enforceable anyway.

And btw, verbal agreements are enforceable - provided one has credible witnesses to stand up to cross-examination (also depending on the legal system of the land). The only problem with enforcing verbal agreements is the cogency of evidence/proof - direct or circumstantial.