The holy Qur'an mentions about keeping witnesses while lending or borrowing money. So, is there any condition for practicing the rule? I mean, if I lend someone as little as 5 or 10 dollar should I keep witness.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

According to the four schools of Fiqh, it is only recommended (not a must) to call to witness whenever one contracts a debt or even writes it down. Scholars based their opinion on Allah's Statement in the Qur'an: "…then if one of you entrust the other, let the one who is entrusted discharge his trust (faithfully)…"[2:283].
As a rule, there is no difference between a big amount of debt and a small one.
Allah knows best.