The effect of non-competition agreements and clauses on the employee

Recently, our management (Free zone company) asked us to sign a document. It mentions 3-month notice period as one clause.

The other being if the resigned employee approaches or starts a business connection (as supplier and buyer) with one of the existing clients of the ex-employer, the ex-employer can file a case against the ex-employee for USD 100 K.

What's the legal validity of this? I heard any document, or contract will not be valid once the employee visa is cancelled.