No, no, no: He didn’t say that you don’t have permission if you don’t steal it, only that you do have permission if you do.
What you said is true: If you take it without permission, that’s stealing, so you have permission, which means that you didn’t steal it.
However, your argument falls apart at the next step, the one you dismissed with a simple “etc.” The fact that you didn’t steal it in no way invalidates your permission, as stealing ⇒ permission, not stealing ⇔ permission, and thus it is not necessarily the case that ~stealing ⇒ ~permission.
No, no, no: He didn’t say that you don’t have permission if you don’t steal it, only that you do have permission if you do.
What you said is true: If you take it without permission, that’s stealing, so you have permission, which means that you didn’t steal it.
However, your argument falls apart at the next step, the one you dismissed with a simple “etc.” The fact that you didn’t steal it in no way invalidates your permission, as stealing ⇒ permission, not stealing ⇔ permission, and thus it is not necessarily the case that ~stealing ⇒ ~permission.
The exception proves the rule. Since he gave permission to steal it, that implies that you don’t have permission to take it in general.