Ḥalfatan is a noun of swearing that reinforces the sense of ālat. Lam negates a following imperfect and places it in the jussive; taḥallal is related to withdrawing from an oath or introducing a qualifying exception.
Lam taḥallal therefore portrays an uncompromising oath. The verse does not independently establish a legal question about whether a real-life oath was subsequently violated; its purpose is to convey the firmness of the declaration within the scene.
Literarily, this creates an image of resistance after the earlier playful encounter. The beloved’s refusal and oath now stand against the poet’s wishes.