Let the "foot" meaning of 足 be A, the "plenty, enough" meaning be B. Will discuss about this topic in the following two sections.
First, 現代漢語規範詞典 第二版 ("Modern Chinese Standard Dictionary" 2nd Edition) suggests that meaning A and meaning B come from different origin, though they share the same character currently.
Figure 1. Meanings of 足 in 現代漢語規範詞典 第二版, page 1758.
It places the "foot" meaning as 足 entry with a superscript 1, 足1, the "enough" meaning with superscript 2, 足2.
Figure 2. Explanation of the superscript.
The English translation of the above description can be:"Those whose form or pronunciation are the same but no relation between their meaning, basically will be in given different entries, with superscripts on their upper right corner. For example, '耳1' and '耳2', '安1', '安2' and '安3' "
Chances are their ancient version characters bear the similar form and later merged together. Seven known variants (i.e. 異體字) of 足 are found here.
Figure 3. The variants of 足.
The hint is given by this blog:
「另有充足、足夠、滿足等的意義。《現漢規範》認為充足等意義的“足”和下肢意義的“足”没有意義上的聯繫，只是形同而已。換句話說，“足”的充足意義並不是下肢意義的引伸。」which in English can be:"足 has another meaning: plenty of, enough, satisfied. 現代漢語規範詞典 treats this meaning as independent from the foot meaning, except that they happen to be the same in form. In other words, the 'enough' meaning does not stem from the 'foot' meaning." (Source)
Among many opinions, even the ancient sound is believed to be something similar to modern ju4 instead of zu2, which is also backed by 說文解字: 卽玉切, roughly j
i + yu = ju.
Figure 4. The Scan of 說文解字 段玉裁注本 足部 足.
and 王力古漢語字典 (Wang Li's Classical Chinese Dictionary (page 1350), (王力 is the name of a late renowned Chinese linguist, who advocated a categorized set of meanings for each Chinese characters in the dictionary, and suggested students to learn classical Chinese with the knowledge of Chinese history.) In the following three-piece scan of explanation, the "foot" meaning is put at first, next the "enough" meaning, third the "qualified" meaning. We can see that the "enough" meaning appears in some really ancient work 「詩經」, dating from the 11th to 7th centuries BC.
Figures 5-7. The Scans of Explanation for 足 in page 1350, 王力古漢語字典.