Is it obligatory to pay Zakat al-Fitr when one possesses less than a Sa'?

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There are two narrations concerning the obligation to pay Zakat al-Fitr when one possesses only a portion of a Sa'. The second opinion asserts that it is obligatory to pay the amount possessed. This view is supported by the Prophet's saying: 'When I command you with a matter, then perform of it what you are able.' Furthermore, because it is a purification (tahara), whatever one is capable of must be paid, similar to purification with water. Additionally, a part of the Sa' is paid on behalf of a jointly owned slave, so it is permissible for it to be paid on behalf of something other than the full amount.