One day, before getting on a rickshaw, I told the driver that I would pay him 20 taka (taka is the currency of Bangladesh) to take me to my destination. He neither agreed nor disagreed to this amount and remained silent. Instead, he asked me to give him charity for his medical expenses. I had no intention of donating anything (as I feared that he was lying and was not in need of any medical expenses). I neither said, "Ok, I will donate," nor, "No, I will not donate anything." I only said, "Let us reach (my destination) first". Upon arriving, when he found out that I would not donate anything, he asked for 30 taka (instead of 20) and said that he deserved 30. When I argued that I had fixed the fare before getting in, he argued that he kept silent at that time (meaning that he did not say, "ok," when I fixed the fare before beginning the journey). Was it obligatory upon me to pay 30 taka? [Or would 20 have been sufficient?]

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

The driver's silence when you fixed the price with him, and he heard you and took you to your destination after that, is considered consent on his part to that price. So he had no right to ask for more. Although silence does not mean consent in principle, it could mean consent when it is accompanied by other indications of agreement.

The Fiqh Encyclopedia reads, "Passive silence is not evidence by itself for consent or disagreement; it is for this reason that the Fiqh rule necessitates the following: 'No statement can be attributed to a person who remains silent, but silence when one should speak is a statement (i.e. consent and approval).' This is so if it is accompanied by indications and circumstances which prove that it is an approval."

Taking you to your destination with his knowledge of the price that you mentioned to him is an indication that he agreed to it in our view, so he had no right to ask for an increase in the fare.

Allah knows best.