Dieter,
Sorry, I was wrong. After some further checking, do it this way:
Turn OFF the signature required check box on the transaction property. Leave your enable logic on the screen field so that the signature is only enabled in the cases you want. You could also add another check box to the screen and enable/disable the signature field based on the screen check box being checked. If you want to check whether a signature has been entered maybe in a validation rule on the transaction to force entry, you can use the EQBOOL function against the signature property comparing it to a constant of "1". This will return true if a signature has been entered, or false if not.
This gives you many options. Work Manager uses the check box on screen with an enable rule on the signature field checking against the check box, then follows up with a validation rule in the transaction that again checks the check box and if true, verifies that a signature has been enetered using EQBOOL.
Sorry for sending you down a wrong path!
Jason Latko - Senior Product Developer at SAP