What is entailed for a husband who swore by "haram" (unlawful/forbidden) that he would not pay for dinner, and his paternal uncle swore the same oath, then his paternal aunt paid?

The preponderant view regarding swearing by something unlawful is that if divorce is intended, it is a divorce; if ẓihār (a form of divorce by injurious assimilation) is intended, it is ẓihār; and if an oath is intended or nothing specific is intended, then it is an oath.

If the swearer did not intend divorce or ẓihār, then he must offer expiation for an oath (kaffārat yamīn) upon breaking it. The breaking of the oath depends on the swearer's intention behind what he uttered. If he intended to prevent the man from paying and that he himself would pay the money, then he has broken the oath. However, if he only intended to prevent the man from paying, then he has not broken the oath.

Swearing by something unlawful is not permissible. The permissible oath is swearing by Allah, the Exalted, for the Prophet, peace and blessings be upon him, said: "Whoever swears an oath, let him swear by Allah or remain silent."