the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
If an oath is invoked as part of a normal speech idiom – like when someone says wallâhî for emphasis when speaking and does not actually mean to invoke an oath – then there is no harm and no expiation.
If the oath about the other person is invoked in earnest, then the one who invokes it has to expiate for if the other person does not comply. The other person is not obliged to comply with the oath about him, nor does that person have to pay any compensation. However, it is preferred for the other person to comply with the oath if there is no harm in doing so, and thereby spare the person who invoked the oath from having to compensate for it.
The one who invoked the oath is solely responsible for it.
Allah says: “Allah will not call you to account for what is vain in your oaths, but He will call you to account for your deliberate oaths: for expiation, feed ten indigent persons, on a scale of the average for the food of your families; or clothe them; or give a slave his freedom. If that is beyond your means, fast for three days. That is the expiation for the oaths ye have sworn; and guard your oaths. Thus does Allah make clear to you His communications, that you may be grateful.” [Sûrah al-Mâ’idah: 89]
The quantity of food that must be given to each poor person is roughly 1.5 kilograms (1/2 sâ`) of food. This means roughly 15 kilograms of food in total. Clothing a person is what is sufficient for a person to wear to prayer. It is not right to give money in lieu of food or clothing.
One may not opt to fast for three days of fasting unless one truly cannot fulfill any of the other above-mentioned requirements for expiation. In case one must fast, it is better but not necessary for one to fast the three days consecutively.
And Allah knows best.
Source: Islam Today